If you look in the install directory for NX, there are 2 folders (STEP203UG, STEP214UG)
Look for NX part files in these folders (i.e. step214ugnullnx50_in.prt)
These are the 'seed' parts that get used when doing a translation.
If you replace the appropriate part with your own template part, it will use it as the seed part.
I would recommend backing up (rename to .bak) the supplied part before replacing it.