Preheat oven to 350°F. Coat a 2-quart square baking dish with nonstick coating; set aside. Cook fusilli as directed on package. Drain and return to pot; keep warm.
Meanwhile cook butternut squash until it's tender enough to mash with a fork (about 8 minutes) in boiling water. Drain water, mash. Stir into macaroni along with the cheese sauce.
While macaroni cooks, melt light butter in a medium saucepan. Add flour and salt; stir for 1 minute. Add milk; stir over medium heat until thickened and bubbly. Remove from heat; stir in cheese until melted.
Toss together macaroni, sauce and squash. Transfer to prepared dish. Sprinkle evenly with breadcrumbs. Bake for 20 minutes or until heated through.