At the moment no CarEditors is existing, but it is planned to integrate a kind of a careditor ingame like the TrackEditor. The CarEditor is on of the stretchgoal on the GeneRacing Kickstarter project, because the effort for the implementation is relatively high.
Planned (and partially tested in a proof on concept) is an .OBJ Importer to allow a full compatible Object Importer which should follow predefined rules.
The object import will be responsible for the visible part. After a successfull import the game related parts could adjusted and the construct of the object fill will be checked.