$100 is not very much in the grand scheme of things, considering you might have to pay a lot more than that to fly on NW (TAs typically come in way south of published fares in Asia and vouchers don't apply to many TA tickets). And, then there's the fact that you pointed out; NW doesn't fly BKK-HKG.

Maybe you can tell us more about your vouchers' terms and conditions. Chances are, I'd then advise you to trade those suckers away in CC to someone that might be able to make better use of them (for example, NW is often the cheapest carrier between NRT and lots of places in Asia). But, maybe you'll be able to do something cool yourself; more info is needed to make that call