Personally, my XML for extra stages looks like this:
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM01.pac" disc="/stage/melee/STGCUSTOM01.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom01.rel" disc="/module/st_custom01.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM02.pac" disc="/stage/melee/STGCUSTOM02.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom02.rel" disc="/module/st_custom02.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM03.pac" disc="/stage/melee/STGCUSTOM03.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom03.rel" disc="/module/st_custom03.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM04.pac" disc="/stage/melee/STGCUSTOM04.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom04.rel" disc="/module/st_custom04.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM05.pac" disc="/stage/melee/STGCUSTOM05.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom05.rel" disc="/module/st_custom05.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM06.pac" disc="/stage/melee/STGCUSTOM06.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom06.rel" disc="/module/st_custom06.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM07.pac" disc="/stage/melee/STGCUSTOM07.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom07.rel" disc="/module/st_custom07.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM08.pac" disc="/stage/melee/STGCUSTOM08.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom08.rel" disc="/module/st_custom08.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM09.pac" disc="/stage/melee/STGCUSTOM09.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom09.rel" disc="/module/st_custom09.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM0A.pac" disc="/stage/melee/STGCUSTOM0A.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom0A.rel" disc="/module/st_custom0A.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM0B.pac" disc="/stage/melee/STGCUSTOM0B.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom0B.rel" disc="/module/st_custom0B.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM0C.pac" disc="/stage/melee/STGCUSTOM0C.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom0C.rel" disc="/module/st_custom0C.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM0D.pac" disc="/stage/melee/STGCUSTOM0D.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom0D.rel" disc="/module/st_custom0D.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM0E.pac" disc="/stage/melee/STGCUSTOM0E.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom0E.rel" disc="/module/st_custom0E.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM0F.pac" disc="/stage/melee/STGCUSTOM0F.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom0F.rel" disc="/module/st_custom0F.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM10.pac" disc="/stage/melee/STGCUSTOM10.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom10.rel" disc="/module/st_custom10.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM11.pac" disc="/stage/melee/STGCUSTOM11.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom11.rel" disc="/module/st_custom11.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM12.pac" disc="/stage/melee/STGCUSTOM12.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom12.rel" disc="/module/st_custom12.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM13.pac" disc="/stage/melee/STGCUSTOM13.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom13.rel" disc="/module/st_custom13.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM14.pac" disc="/stage/melee/STGCUSTOM14.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom14.rel" disc="/module/st_custom14.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM15.pac" disc="/stage/melee/STGCUSTOM15.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom15.rel" disc="/module/st_custom15.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM16.pac" disc="/stage/melee/STGCUSTOM16.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom16.rel" disc="/module/st_custom16.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM17.pac" disc="/stage/melee/STGCUSTOM17.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom17.rel" disc="/module/st_custom17.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM18.pac" disc="/stage/melee/STGCUSTOM18.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom18.rel" disc="/module/st_custom18.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM19.pac" disc="/stage/melee/STGCUSTOM19.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom19.rel" disc="/module/st_custom19.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM1A.pac" disc="/stage/melee/STGCUSTOM1A.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom1A.rel" disc="/module/st_custom1A.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM1B.pac" disc="/stage/melee/STGCUSTOM1B.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom1B.rel" disc="/module/st_custom1B.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM1C.pac" disc="/stage/melee/STGCUSTOM1C.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom1C.rel" disc="/module/st_custom1C.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM1D.pac" disc="/stage/melee/STGCUSTOM1D.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom1D.rel" disc="/module/st_custom1D.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM1E.pac" disc="/stage/melee/STGCUSTOM1E.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom1E.rel" disc="/module/st_custom1E.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM1F.pac" disc="/stage/melee/STGCUSTOM1F.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom1F.rel" disc="/module/st_custom1F.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM20.pac" disc="/stage/melee/STGCUSTOM20.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom20.rel" disc="/module/st_custom20.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM21.pac" disc="/stage/melee/STGCUSTOM21.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom21.rel" disc="/module/st_custom21.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM22.pac" disc="/stage/melee/STGCUSTOM22.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom22.rel" disc="/module/st_custom22.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM23.pac" disc="/stage/melee/STGCUSTOM23.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom23.rel" disc="/module/st_custom23.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM24.pac" disc="/stage/melee/STGCUSTOM24.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom24.rel" disc="/module/st_custom24.rel" create= "true"/>
<file external="/private/wii/app/RSBE/pf/stage/melee/STGCUSTOM25.pac" disc="/stage/melee/STGCUSTOM25.pac" create= "true"/>
<file external="/private/wii/app/RSBE/pf/module/st_custom25.rel" disc="/module/st_custom25.rel" create= "true"/>
Just swap out
/private/wii/app/RSBE/pf/ to your own path if different, and they should work.