Apart from GP3edit and GP3Master, which program can be used to insert insert GP3cars into the game. The thing is I want to assemble my own carset using other people's cars for my personal use in the game. As far as I know I can only import a full carset in the game, but since there is none for the 2005 season...
With GP3edit you can add several cars in the game.
Go to the "CARS" tab and in the Bitmap Options click in the LOAD botton, that way you can add diferent car livrerys from different authors in the same carset.