Conversion help

It's been a long time since I've played GP4 but I've just returned to it today.

After playing F1 challenge for a while, I wanted to try and put some of the cars I've played into this game, but all I can find are tutorials for converting Gp4 cars to f1 challenge.

Is it possible to do it the other way around? (Not doing public releases, just for personal use)

Thanks.