Originally Posted by bocastephen

This is strange, I've not had this come up before on my own tickets. Have you tried to add it yourself online?

I would just carry your *G UA membership card with you in order to get your benefits along the way, then send in the boarding passes to get the miles posted after the trip assuming there ends up being no way to add your FFP. Also, as you transition from your UA flights to the other carriers, ask them to input your FFP, it might go in on their systems without a problem, so you'll only need to deal with the United flights.

I also wouldn't take the word of one agent at face value - haver you done the HUCA? Called the MP Service Center to ask for their assistance?