Preheat oven to 350°. Cook shells according to package directions for al dente., Meanwhile, in a large bowl, mix mozzarella cheese, turkey, stuffing and green onions. In a small bowl, stir sweet potatoes and, if desired, mix in chili powder., Drain shells; fill each with 2 tablespoons stuffing mixture and 2 teaspoons sweet potato mixture. Arrange in a greased 11x7-in. baking dish; sprinkle with Parmesan cheese. Bake, covered, 15-20 minutes or until heated through. Serve with gravy.