Not sure on a UA stock, but, I just flew a revenue ANA fare in business (Z), and was able to add my TK FFP while buying direct from ANA (on ANA stock). Did you try to get your NH PNR from UA, log in to MMB on NH, and try to add it online? Their UI is pretty simple and easy to use.
Other FT'ers with similar experience should chime in: I am interested to hear that too!
P.S. If it is a LH M&M booking, I have always been able to add my TK number to my booking, by calling the service center, or while checking in online!