You could just edit the raws like mentioned, the cannibalism tag on ethics department, I recall, was the one you needed to change.
Skin everything, recycle their meat as masterpiece meals, sell to traders. Make clothes out of the leather for your dorfs.
Recycling, it is a great thing to do, even with cannibalism.
I should change the tag myself. Dozens of invader corpses, have to get rid of them somehow. Might as well make a profit and return to the art of 40d, decorating my skull totems with bones of fallen enemies.