When making chicken stock can you combine cooked bones with raw ones?

Yes, you certainly can. But I would recommend is to put the wing tips in a roasting pan along with some cut up onions and carrots. Brush this with oil and roast in the oven at 450F until the onions and carrots begin to carmelize. Use this mix along with your along with your carcass and it will give your stock a nicer color and flavor.

Yes, absolutely. I do that all of the time, saving up bones from roasted chicken in the freezer to use later to make stock, adding fresh chicken parts if I don't have enough of the cooked frozen ones.