I'm going to say rubbish, at least the part that it's as easy as just adding a AAC file.
I'm sure romhacking the DS versions of Guitar Hero is possible, just like the PC versions were. But the limited fanbase for it and the requirements for the tracks (smaller and lower quality to fit in cart size) don't make it a worthwhile venture. If it's possible, you'd also have to hack in your own note charts for the music, the game itself won't generate it for you.