Stove Top doesn't really have to be "cooked", so what I would do is cook the onions and celery in the amount of butter for the quantity you're making, then add chicken stock instead of water and bring that to a boil. Then I'd dump it and the stuffing mix in the slow cooker and just let it stay in there on low until you're ready to serve it. I think it would be fine, and it's actually a great idea! I think I'll try that next Thanksgiving so I don't have to worry about putting the stuffing in the oven to keep it hot.