But - you got to make sure those bees you add do not kill the queen, which will at first be foreign to them.
However if adding brood to a weak hive, mostly you will also have to add bees. Because the weak hive may already have as much brood as those bees can cover, and if you add more brood the bees cannot care for it and the brood may die. Very common trap for new players unfortunately.
There is always a risk when adding bees that they will kill the queen. Here is what I do to minimise that risk. I find the queen in the weak hive and put the comb she is on on the outside of the brood nest with the queen on the outside side of that comb. Then any brood and bees get added at the other side of the brood nest.
Course doing that you still have to keep the integrity of the brood nest, ie, end up with a compact brood nest that the bees can properly cover.
Other thing to be aware of is some of those added bees will just head back to their parent hive so you need to try to add young bees that haven't flown yet, plus add more than you need to end up with to allow for drift. Also if it's robbing season take precautions to prevent some of those older bees returning to the weak hive to rob it.
Do all that properly and you can expect a very high success rate.