Giving hives an extra comb or two of honey is routine among commercial beekeepers. That said, I agree with River that you shouldn't combine a weak hive with a strong one but combining 2 (or more) weak hives to make a strong one has never worked for me in a commercial setting. I just shake the bees out in front of another hive and start a new hive in that equipment the following year. Chances are that the factors that made them weak in the first place would still be there after they are combined. You'd just have a poor hive with more bees in it most of the time. I'd rather salvage now, or as some call it, taking your spring losses in the fall