From a quick google is doesn't seem possible to convert from max files to .psv. However, I imagine if you could get the file onto a ps2 memory card and then use an
adapter to transfer it to the PS3, you'd be all set. The problem of course is that you'd need a PS2 and an adapter, and I assume you don't have a PS2 or you wouldn't have this problem.
However, if you can find a friend with a PS2 and an Action Replay Max CD, and got a hold of that adapter, I think you'd manage.
On the other hand, unlocking the cars the long way is pretty rewarding, and if you can ace the Driving Revolution section (easier said than done) you'll have unlocked all the cars anyway.