If it's a round trip ticket, the rest of the itinerary will be cancelled as soon as you miss a leg (unless you have a really good excuse for doing so). This is why you can't skip NRT-TPE and then fly back TPE-NRT-JFK.

Call NWA, tell them you want to stop over at NRT, and they will tell you how much it costs to do so. It will likely be less than $300.