Guitar Hero games these days have the ability to create your own music in the studio, not sure if Rock Band has it.
Unless you meant actually adding in the music via MP3 and it works out the buttons. That would be a wicked idea, but seems to futuristic to see it anytime soon. Would be cool because then I could put my bands stuff on there (as I am not smart enough to create it in the game because it is friggin difficult to create a song )