In my experience, crews are not permitted to "build" approaches, they must be in the database from an approved supplier, and arent to be modified.
Sure, you can build an approach using whatever waypoints you like but during a GPS or RNP approach you are asking the aircraft to fly a track determined in space by GPS.
In fact, some approaches ask you to deselect nearby navaids to prevent the chance of any radio updating so you dont inadvertantly get EPE exceeding the limit for the approach.