I've asked around at a few sites about this. Unfortunately I don't have the time to learn how to convert cars as there is a decent one in GTR2, but if I had the talent I'd use the one from NFS Shift.
Hopefully someone will at some point as I'd love to have it for STCC. A Chevy Cruze is on the way and we'd just need a Vauxhall Astra to complete it.