Heat oven to 375°F. Spray 11x7-inch (2-quart) glass baking dish with cooking spray. Cook and drain pasta shells as directed on package. Rinse with cool water; drain.
In large bowl, mix 1 cup of the Parmesan cheese, the pumpkin, ricotta cheese, egg, 2 teaspoons sage, 1/2 teaspoon of the salt and 1/4 teaspoon of the pepper until blended. Spoon about 3 tablespoons pumpkin mixture into each pasta shell. Place in baking dish.
In 2-quart saucepan, melt butter over medium-low heat. Stir in flour and remaining 1/2 teaspoon salt and 1/4 teaspoon pepper. Cook and stir until mixture is smooth and bubbly. Gradually add milk, stirring constantly, until mixture boils and thickens. Stir in nutmeg and remaining 1/2 cup Parmesan cheese until blended. Pour sauce over shells.
Bake uncovered 30 to 35 minutes or until golden brown and bubbly. Garnish with sage leaves.