to get a stopover you need to have a r/t ticket, stopovers are not possible on one ways IIRC
so you'll need to adjust the already existing PNR. you will be allowed 4 segments to get home. you could do something like SYD-ICN-JFK-IAD on Asiana and give yourself a stopover at ICN.
Thai Airways SYD-BKK is another option, continuing BKK-NRT-IAD after a BKK stopover.