We don't recommend this. If you're using raw sweet potato, it will require a lot more time than your other veggies to cook. If you add everything at once, you'll end up with cooked onions, peppers, and zucchini, but crunchy, unappetizing, not-fully-cooked sweet potatoes. If you're in a hurry, stab your sweet potato several times with a knife and microwave it for about 3 minutes (depending on its size) and then peel, slice, and add it to your pan. Additionally, as one tester mentioned, the kale should be added last so that it doesn't completely wither and wilt away in the pan.