If you want to add a model to ns2 you can use blender or 3dsmax (save the file in the 3ds format -2009 If I recall correctly- and use the tool called "builder" in the steam "natural selection 2" directory.
If you want to import ns2 models in a different software, maybe you can use 3ds max or blender but I don't know how since I suck with 3d softwares (exept Catia but it's not made for gaming )
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If you want to import ns2 models in a different software, maybe you can use 3ds max or blender but I don't know how since I suck with 3d softwares (exept Catia but it's not made for gaming