I share your sympathy for the dogs, but once you clear the keep they are obliterated anyway, so I usually get over it.
Well, assuming you can get over the ethical question of killing the dogs, it is possible to get the experience both for making the stew and killing the Umber Hulks. Here is what I do:
SPOILER
Make the stew and place it in the tunnel just as Galuf has indicated above. It will take a few moments for the hulks to arrive, so use that time to throw a cloud kill spell in the corridor. Nip around the corner so that you are out of sight of the tunnel, so that you don't get nailed with the confusion spell the hulks throw. As the hulks enter the corridor, you will see their corpses pile up as they enter the gas cloud. Occasionally one or two will survive for an extra round, so make sure you are out of sight.
Of course, you can fight them the manly way, without using the cloudkill spell, but in that case your characters will need to be protected from their confusion spells (the cleric spell Chaotic Commands works well for this.)