can we get an option for replacing stgcustoms into normal stg's of our choice
That would be difficult, since the st_custom rel files use the sequence 38 A5 00 00 38 80 00 00, which also appears elsewhere in the file. The code would have to figure out (probably by filesize?) which .rel file it is, and then give you that one instead. Also, I'd have to add code to let you upload files in order to pull that off.