I agree with the above advice. However, you might be able to get a general idea of what to expect on JAL's site. Click "More" to the right of the origin/destination city fields in the first flight booking page, then "For other itineraries" under the origin/destination fields of the next page, and finally "Search multi-city/stopover itinerary" at the top of the next page on the right. Try various durations for your Tokyo stop and see how the fares you find compare with going directly to BKK. If nothing else, you'll be better informed about the options when you talk to agents and will be able to compare what they find with what you know you can do yourself.