Hello AstronomerSolar.
Its pretty easy to import models into Universe Sandbox. but keep in mind this is still in beta and not yet finished.
Things you'll need:
A model.
A 3D software: Blender or 3DS max with .x plugin
Universe sandbox.
I'll be using 3DS max cause i dont know how to use blender, its annoying.
start 3DS max up (start blender up)
Here is my 3D model. (same as blender)
Export as OBJ (in blender, export it as .x or as OBJ) and save it to your Universe sandbox Media folder. (C:\Documents\UniverseSandbox\Media\)
Spawn anything, i'll be using a comet
click it, second tab and select ''Open Media Folder''
select your model and you're done!
Enjoy.
P.S: add me on steam, Intensive Mono Unit or MonoNova, both are the same.