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)
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Here is my 3D model. (same as blender)
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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\)
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Spawn anything, i'll be using a comet
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click it, second tab and select ''Open Media Folder''
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select your model and you're done!
Enjoy.
P.S: add me on steam, Intensive Mono Unit or MonoNova, both are the same.