Numerous games allow you to have 'custom soundtracks' by importing your own music into it, so I can see this happening. They'll (Rockstar) most definitely release packages containing several new songs for each radio station on the PS3's online store, on the Xbox's Live service, the same for the Wii, and just as a download for PC. However, the songs might not flow seamlessly and it might be difficult trying to get the radio station host to describe the new tracks you imported. If you use the possible packages from Rockstar, then they'll most probably record new voiceovers for the material.