If you want the Exact (BREP) Geometry, from an STL Body, you have to use the Reverse Engineering tools... Rapid Surfacing, etc...
I suspect you can machine directly a Faceted Body, with NX CAM
I don't know if a special license is needed, but I'm running NX 188.8.131.52, and there is an option to select a facet body in the "filter" option of the rough offset command.
Try changing the Deviation and Stepover settings. Also, from your image, the model appears to have some very small 'blends'. This might be problematic. Also try setting the Offset Distance to zero (0.0).