I'm purchasing two J tickets jfk-bkk.

When I price it out as multiple cities Jfk-bkk, bkk-han, han-jfk the price doubled regardless of the dates in japan.

On JAL on most itineraries this stopover adds only $100 to the fare.

Can I buy JFK-BKK then call ANA to fit in a tokyo stopover for a minor fee like JAL? I realize some days may not have promo fares availability, but I have plenty of flexibility.