the easiest way is to export a STL file and reimport it. There is no command like "convert to facets"
Facets generally are used in various ways in NX: Automatic Lightweight Representations in Piece Parts and Lightweight Drawing views, Product Outlines, STL's Reverse Engineering Processes, Facet Machining, CAE Meshing, and runtime generation for graphics display.
So basically STL is a means to describe a facet mesh in a neutral format, but not appropriate for all NX Downstream processes.
Hope that helps.
You can also try to use the following command:
ug_inspect.exe -extract_jt <filename.prt>
from a NX command prompt windows.
It extract the JT file (so infact the NX faceted representation...) from UG part...
I don't think so, but, since it's very easy to create a command line, you can create a list of parts and then add the command launch.
You could use the ugtopv command line translator to create JT's from a list of parts. Type "ugtopv -examples" in an NX command prompt to find the following amongst the list:
generate jt component files for all .prts or .bkms in the text file