There are modules/modifications you can add to spacecraft, but all actual ship hull creation is out-of-game. You can create entire new ships, but you will need to create the 3D models and such on your own (no in-game systems for doing this; you would need Maya, Blender, or whatever your model creation software of choice is).
Now, there's 3 distinct gameplay types which need to be taken into account:
1. Persistent World/MMO/Official Servers: These would then be submitted, and be approved/rejected for addition into the game. They would need to be pretty high quality in order to be approved; on the level of those already existing. It has been stated that they want a relatively low number of ships in order to keep the ships themselves relatively iconic.
2. Persistent World/MMO/Private Servers: These would be whatever the private servers' admins decide upon.
3. Singleplayer: Just add correctly formatted ship files and such to your game.
Most of the customization will take place with the various modules, not through changing the hull itself.
As a little side note to your last bit, you will be able to have player crews, but also hire NPC crews of varying skill/costs.
Important Note: While I am a dev, A: my job is writing graphics shaders and B: I haven't actually started work yet. So don't take my posts as anything more than a layperson with publicly released info, because that's exactly what I am. And even after I start, I work on the graphics, and so anything gameplay related I say is liable to be wrong as well as certainly being liable to change at any moment, and will be based solely on beans which have already been spilled.