Someone started a tutorial
here but never finished it.
There's a thread about a newer-ish zone importing tool
here with some nifty screenshots from people playing with it.
That tool is somewhat old and crappy now, but I'm probably still the main person who would consider making model-importing tools at this point. It's a pretty niche subject, pretty much never more than one person interested in doing any custom model stuff at any one time.
If you've never done any 3D modeling, I'd suggest downloading Blender and finding some tutorials for basic stuff on youtube to start off. Even if you do have some experience with Blender/Maya/3DMax, I'd suggest starting off by making and importing weapon models, since they are simpler and quicker to get something decent you can look at in-game, but also introduce you to most of the same concepts and concerns that would also apply to zones. There's a
thread about another tool for importing weapon models.
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