Hi folks - I'm usually on the AS forum but booked via MP here so could use some help.

I booked award J on ET from JFK-ADD. Award space has opened up ADD-JNB, and I'd like to add the ADD-JNB leg as my final destination is in South Africa. Both online and with 3 phone agents, it seems like I can only create a multi-city itinerary at +35k Y or +90k J (and to be clear, I do not wish to make a stopover in ADD, but book a single one-way itinerary where I retain the JFK-ADD and travel onwards to JNB).

Several other days that week have availability ex-NYC to JNB via ADD bookable at the 88k level. Is it really true that I cannot change an existing itinerary to do the same? If not, are there certain magic words I need to ask for from the phone agents? Bit at my wits end here and would be surprised if this is impossible, but also am not familiar with the specifics of UA's rules.

TIA!