SpaceSimCentral

I wanted to use models from the game in blender but I don't know how to import or convert it properly. I also cannot find something useful on the Internet. What's that format and how to use it in 3D software (except Pioneer, of course)?

There's no tool to convert a compiled sgm back to a readable format. It's our own optimized binary format.
But all the ships have their original collada files on our github, since they are compiled only upon packaging/releasing the game. And Pioneer can read collada no problem.
You can find them in the same data/models/ directory