Hello houston144,
3rd party addon creation without the direct approval and cooperation of Vstep is not possible, I'm afraid.
You cannot put your own creations in the game. And as for reskinning, there are only a handful of ships that can be reskinned, most of which non-player vessels.
If you create something of good enough quality, Vstep will consider a cooperation with you to include it in the game, but since SS08 is no longer under development, this would only still apply for SSE. And no hard guarantees as it also needs to make sound economic sense. Vstep has chosen to stay in control of the content as opposed of other games where this often results in an abundance of substandard content, often with a lot of incompatibility problems between players with different addons.
furthermore, the tools are simply not available for someone to do this on his or her own, it requires specific, in-house used programs that the general public cannot obtain. Hence the need for Vstep to do a lot of that work. Creating a model is one, but implementing it, you can't do that yourself.
The new venue here is the CCC where model making is poured into a competition form, but there as well, it's just for SSE and only in cooperation with Vstep. If this will change in the future to a more free form of content creation is unknown and for the near future, unlikely.
But the same would still apply between those that do or do not have certain vessels in their game, as it also does with Ship Sim 2008 already. If player A has an addon vessel that player B does not have, they cannot play on the same server, because the server would reject the player without the vessel needed to connect. So the vessels you can choose from are server-defined. If you haven't go it, you can't play on that server.
I hope this answers your question.
Kind Regards,
Fred