Have you ever heard about ripper 'Riva'?
He is the one who is ripping models form Forza 3.
Every one is downloading his ripped models & converting it to GTA IV or any other game (even I'm doing the same)
If you don't believe me try this http://www.mm2x.com/mm2riva/forza3_cars.html
but the texture used in Forza 3 are very ugly & converting car's is not easy it is more harder then you think
And last thing Zmodeler 2 don't have Forza 3 filter.