I haven't gone down that road yet, myself. Whatever modeling software you use for modeling will have to be able to export to an .fbx file. Then you can use the Launcher to convert that file to something the game can see. There are editors available via the Launcher for viewing existing models.

Honestly, I haven't done any importing and I haven't seen anyone else do this yet. I'm still nailing down how to create Addons, vs mods. I think since the game is still in development, people are probably staying away from importing their own models for now until everything is more set in stone. I personally would love to see more realistic NASA style models in the game such as a the ISS, the HAB, maybe even a working SPHERES that follows you around in zero g.

You could contact space x or nasa, show this game to them, and ask for some of their model's :) as im sure they use it in their simulation runs alot :). heck it's even a good way to get input on what they would do..most of those guys LOVE exchanging knowledge.