I looked into this itinerary last year, as a possible mileage run, as I was heading to Fiji anyway. I only looked at the individual prices for ATL-ICN round trip on Orbitz, and ICN-NAN round trip on Koreanair.com. That combination gave reasonable prices.

I think the way to do it is to work directly with DL or KE, or a travel agent.

In then end it was not going to work the days I wanted to travel. This, combined with the fact ATL-ICN-NAN adds two additional days to the travel time compared to ATL-LAX-NAN via NZ or FJ out of LAX, meant I went FJ.

Personally, the best way to get to NAN is via LAX on NZ. They fly 767-300ERs on that route, so you not only get the widest economy seat on the market, but by flying NZ you also get the best economy seat on the market (Recaro's CL 3510). Throw in NZ's inflight service and you have the best international economy flight experience outside of SQ's 777s.