No it's not possible. The best U could hope for is to be able to modify the existing cars, and that is highly unlikely as things stand now. If a Pro type device is ever released then that would be a different story, but even then it would not be possible to create cars for GT3. What boombexus was telling U, is that the car has to be on the game disc, which the 911 and Diablo and the others are. But even if we could figure out how to code a new car for the game we couldn't add it to the disc.