I imagine you could but you would need to mix it in which would probably stir it up more and you would accomplish exactly the opposite of your desired effect. I don't mind a bit of haze as long as it tastes good.
Chris, do you really need to add stuff to the keg? It's only been three weeks and it's getting colder, so IMHO you just need a bit more patience. Should clear in 2 or 3 weeks.