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I've found the "threading.scad" part of the BOSL library to produce a wide range of screw threads well.
I'm not expert, and it doesn't define UNF threads itself, but if you know the characteristics of the thread profile, I'd bet you can produce the threads you want with this library.

Edit: I looked up the threads on Wikipedia, and checked threading.scad and I think it'll do what you want.
Just look at the trapezoidal_threaded_rod() module and I think you are golden.
If you struggle, and I'm awake :-) I'll try to help more.