Give them a frame of eggs/young larva from a good hive. Do that each week for a few weeks and see if they make cells. You may also have to give them a transfusion of capped brood with the adhering bees if they've dwindled way down. I'd be real hesitant to combine at this point. As you stated, they may kill your good queen. If that seems like too much work for this hive then just shake them out a little distance from other hives and remove the box so they can't return; they'll find their way to other hives and join them.