Preheat the oven to 350 degrees F. Prepare the macaroni and cheese as directed on the package. Stir the tuna, mushroom soup, additional milk, and half of the Cheddar cheese into the pasta mixture. Mix well. Spoon the pasta into a casserole dish. Combine the remaining Cheddar with the bread crumbs. Evenly sprinkle the crumb mixture over the top of the casserole. Place the casserole in the oven and bake at 350 degrees F for 15-20 minutes or until the crumb mixture is lightly browned and the pasta is bubbly. Serve hot.