An air carrier crew accepted a PAR approach to NXX (Navy Willow Grove) because no other authorized precision approach was available in the aircraft's FMC. Their carrier however; did not authorize PAR approaches.

Narrative

The Captain and myself were dispatched on charter reposition to NXX. The weather at dispatch time was IFR and we had BWI as an alternate. Four approach charts were provided in our charter kit; RNAV (GPS) 15; RNAV (GPS) 33; NDB 15; and PAR ALL RWYS. Weather upon our arrival was ceilings 700 BKN 1000 OVC with 5 miles visibility. As we reviewed our options for approaches and chose RNAV (GPS) 15 we discovered the RNAV approaches were not in our FMS database. The only approach in the database was for NDB 15. About the time all this was occurring we were checking on with Philadelphia Approach who told us to expect PAR 15 approach at NXX; Willow Grove. We accepted and executed the approach and landed. After conversations between the Captain and me and a review of operation specifications I realized we were not authorized to do PAR Approaches. Furthermore; we may not have been legal to be dispatched to NXX in IFR conditions since the only approach in our FMS database was an NDB approach. Even though we had charts for RNAV (GPS) 15 & 33; our limitations state 'Do not use Integrated Approach Navigation when any altitude constraint specified by the approach procedure for a final approach fix; or waypoints between the final approach fix and the runway; have been modified by the flight crew.' Which meant we could not have manually build the approach. Neither Dispatch or the flight crew determined at time of departure we may not be legal to depart. At no time did we feel that we compromised safety as we continued; and the final out come was the completion of our assigned charter. Event occurred because of incomplete resources. I should have known definitively that PAR approaches were not authorized and with no other options diverted to our alternate. However; we were lead to believe we had other options; approaches; only to discover at the last minute we did not.