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 and 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.