Select the DOF that you want to reduce to into the a-set. This is done via ASET bulk data entries.
See Section 10.3 of the User's Guide.
The Users Guide is in the NX Nastran documentation. If this is a bundled NX installation, see the NX docs and navigate to CAE, NX Nastran, NX Nastran User's Guide.
NX Nastran documentation is also available online at: