You can build whatever you wish for approach, except manoeuvres like procedure turns, DME arcs etc. These can be approximated using suitable radial/distances.
If there is a VOR or even ILS approach in the FMC, which mirrors the NDB profile it is acceptable to use this. If you construct an approach you would most likely not be allowed to use LNAV, but HDG SEL and monitor raw data. You should monitor raw data at all times anyway.