Have you tried GMP-HND-ITM? If you already have ICN-NRT and can't change it though, maybe get HND-ITM on avios. Will require a change of airports though. There are a lot HND-ITM flights, but only 2 NRT-ITM flights. There are 3 HND-KIX flights as well, but no NRT-KIX...unless you could JL codeshares operated by GK (Jetstar Japan).