I might try to figure out the Tracklist Modifier so we can change the added stages' music as well.
There's a program called BCSM-GUI that makes editing songs obnoxiously easy. It works with .mp3's, not just .wav's and it automatically puts the song on your sd card where it needs to be after it converts the song to .brstm.
if you want to try it out, here's a link to a video tutorial with a download link in the description: