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