According to this post, the developer for Zmodeler said he is releasing an import tool for GTA V this weekend.
Now I'm no developer myself, but I believe this will allow us to import vehicle models into GTA V! If so that's awesome.
Edit - Found an image that he posted on the forum, the image shows a ball model attached to the back of a Buffalo 2. This is part of the testing that he has done so I added the image for anyone interested: