The models start as a source file in something like Blender. You export a mesh from this and frame the mesh with a config.txt file and all the other red tape.
Normally content creators do not release the original source so making changes to the mesh is difficult. For your own use you can create a second mesh with the passenger attachment points on and sort of merge it in with the original. However you need to know what attachment points are on the original to avoid using the same names. This is not normally considered 101 stuff but there is a write up in the wikibook.
Have a read of the CCG (Content creators guide) to see how it all works.
