You should use the new NX 11 convergent modeling. Therefor you must convert the facet body in a convergent body with 'Convert Facet Body'. After that you can use the normal body creation and edit commands like, extrude, trim body, unite or subtract.
so I've imported .stl and now I have Convergent Body. But the only thing that has changes is an entry in Part Navigator. And I can not interact with the body.
Whats the workflow with this Convergent stuff? Some hidden tools? Command search findes nothing if I input convergent.
Here are some video that have been posted to help explain what can do.
and another one
Just remember that this is the first release of Convergent Modeling technology and we are going to be making a significant investment in this in upcoming releases of NX.