Can I make boxed potatoes and not cook them for a few hours later?

If you mean prepare certain packaged dried potatoes (such as Au Gratin or Scalloped) the answer is, yes. Just follow the directions as listed (but not yet ready to place into the oven yet).

The potatoes will rehydrate, but for food safety’s sake, you should lightly cover and keep the dish refrigerated until ready to bake. Keep in mind that the finished preparation may vary somewhat in terms of cooking time and results from if you had placed the preparation directly into the oven, but you should still expect a desirable result.