Hi Simon, and welcome to the forum!
There is currently no means to put any ships in the game, as a 3rd party addon maker. Only VSTEP can do that. And tools to make environments are not available at this point.
VSTEP does however encourage talented modelers to make ships. But the tools needed to do a good job often are very expensive. The best tool for the job is 3D Studio Max.
But there are a few free programs that could produce a good result. Gmax and Truespace are such programs.
To make a possible addon vessel suited for the game, certain guidelines have to be followed, I included them in this post.
But this is all a very difficult and time consuming process, there is no easy way to do it. It takes a lot of time and dedication.
Hope this answers your question.
Regards,
Fred