deufeufeu made a translation with some songs from the expansion
check his site

 

1. There is a hacked dump of Band Bros, where the original songs are replaced by the addon songs. This lets you play it without a slot-2 cart, but you can't have both the original & new songs at once
2. There is a dump of the addon, but I'm not sure if it works with the 3-in-1 or not. I know it works with older GBA flashcarts in slot-2 with band bros running from slot-1, you'll have to try it

Also: There are tons and tons of custom songs out there people have made for the game. You can play them in one of 3 methods:
1. Replace the original songs with custom songs in the .nds file
2. Replace the addon songs with custom songs in the .gba file
3. Transfer the songs into your savegame file