When I buy a box of scalloped potatoes (on sale!), I add an extra raw potato, cut in thin slices, before cooking. I also add 3/4 cup more milk than required. It makes more scalloped potatoes, so I can feed more people!
when wanting to stretch a box of scalloped potatoes, I prepare as to the directions on the box and just before putting them in the oven, I stir in drained tender-crisp cooked broccoli & carrots. I've used mushrooms and onions before as well. I'd say the possibilities are endless as to what you could also put in extra, maybe some chopped, cooked meat.