While I definitely prefer the flavor of fresh carrots, frozen carrots will definitely work for this recipe. It's possible to even cook them from a frozen state. Simply measure out your desired amount of carrots in a bowl and toss with them with the oil. Lay them out on a baking sheet and sprinkle with salt and pepper. Then cook in the oven as directed in the recipe. Just note that since they are frozen they may take a little longer to cook.