I can't find the starbases in the game files, but it can be done. At least with a mod.
That's how starbases are made for custom races - You make the ship and save as a template, which makes a 3D model you can use.
I would assume the reverse can be done, where one would set a starbase model as a ship. If I get time I'll find the easiest way to do this, I've gotten enough practice with shipsets and mods 