NX supports the use of NX Nastran external superelements via its assembly FEM capabilities. The external superelement is represented via an OP2 file that is created by a component reduction solution (NX Nastran SOL 103 Superelement in terms of NX supported solutions).
I suggest that you search the NX online help for the following article:
Using superelements in an assembly FEM
It will give a brief review of superelements in NX, but the right hand column "Learn more about" should have many more articles describing the NX processes that support superelements.
Simulation Product Management
Product Engineering Software
Siemens Industry Sector
Siemens Product Lifecycle Management Software Inc.