I think there is a code that allows you to give track slots to stages by taking it from one to the other. no clue though. if anything, just take a Brstm file straight from the game's files and rename it to replace a different song.
Sorry for the late reply, but I didn't think I was going to get any in the first place. I would do that, but my thing is that I want to have a massive amount of options for each stage. For example, if I really like a song, I want it on a few stages, or at least possible on some, but sometimes it just doesn't fit with the limited number of slots some stages have. The menu only has four song options, but there are plenty of brstms I could use in the menu if it just had the room.