Originally Posted by
jsloan
I have no doubt that what you (and
WineCountryUA) describe can be done.
However, my reading of the OP's situation is as follows:
OP has a ticket from XXX-LAX-DEN, ticketed via NH (cash or miles, doesn't matter)
OP's actual destination is YYY.
OP wants to purchase a ticket, LAX-DEN-YYY, that will serve as an EXTRASEAT from LAX to DEN,
but will be a regular ticket from DEN to YYY.
(In addition, OP will have a lap infant, which is going to complicate things even more).
I do not believe that this is possible as stated.
OP: I agree with
WineCountryUA that your message is somewhat ambiguous. If I haven't described your situation well, please feel free to add additional details.

This is correct. DEN-YYY would ideally be my final destination and is not ticketed currently (was planning on driving). The LAX-DEN is my only leg in Y and I am seeing LAX-YYY <$100. It would be worth that price for me to have a second seat LAX-DEN and a 45m flight instead of a 4.5h drive to YYY
The lap child is currently on the BR XXX-LAX leg and not on the LAX-DEN leg so there wouldn’t be duplicate lap child legs.