You can always try forcing it using the multiple cities option for ATL-ICN on day 1 and then ICN-NAN on day 2 or 3, depending on the timing for the connection and something similar on the return. Depending on the ticketing carrier they may not publish fares on the ATL-NAN routing, so it would be trying to assemble 2 tickets back to back which may add to the cost.
KE publishes fares on the route using only their metal (some involve a 24+ hour connection in ICN which indicates to me that ICN-NAN is not operated daily) and you'd have to find your way to ATL. That being said, that fare may be more expensive than the multi-stop that you're seeing which is why the various sites aren't showing it.
Finally, the carriers' sites are known to skew the search options to keep you on their metal as long as possible to maximize their revenue. You are likely experiencing this phenomenon as well.