Adding a flight to an existing reservation?
 

I think I know the answer to this and this has no doubt been asked before so please move as needed.

I am flying xxx-yyy-zzz in First in a month. I want to add zzz-aaa with a 1.5 hour connection. When I try to change flights it doesn't offer my original routing with the addition. All offered flights are 1000 more while I can purchase a separate ticket for zzz-aaa for 125. Should I just buy the separate ticket or is there a workaround? I add that my wife is already on the xxx-yyy-zzz-aaa flights, first flight is on HA , and both tickets are on 016 stock. TIA

Yep, no workaround as they can't just add on a segment to ticket without doing a refare using current fares (using xxx-aaa fares, with your xxx-zzz fare simply used as a credit for new fare) and a ticket exchange and that can be quite expensive. There is some risk when booking segment as separate ticket -- if there is a schedule change and MCT is too short, they may not let you switch flights with no fare diff, and IRROPs handling can be more of a challenge (but suspect they should generally work with you when both tickets are UA tickets).