Find some export/import plugins for ZModeler which can read some game carshapes. Edit the shapes as based on GP4 shape format thru ZModeler, then export it using GP4 format.
Converting from one game's carshapes into another was never easy. You need lots of works to welds and splits some vertices so can be grouped into specific exported format. The easier way is design a carshape from scratch so you know exactly the structures of polygons were you made.
There's no perfect tools for converting from game's carshapes to another. Most carshape designer are building carshapes based from some pictures taken in different angles of the car.
If you don't want to build a GP4 carshape, pick from original shapes and try editing several vertices. However, avoid to publish a modified GP4 carshapes. Use it for your own pleasure only.
In reality, doesn't matter who's
right but most important is who's
left.