Preheat oven to 350°. Cook spaghetti according to package directions. , Meanwhile, in a Dutch oven, melt butter over medium heat; add onion and pepper. Cook and stir until tender, 5-7 minutes. Drain stewed tomatoes, reserving the liquid, and chop tomatoes. Stir in soups, tomatoes and sugar if desired. , Drain spaghetti; add to sauce mixture with Velveeta and chicken and if desired, reserved liquid. Transfer to a greased 13x9x2-in. baking dish. Cover and bake 20 minutes. Uncover and bake until bubbly, 5-10 minutes longer. If desired, sprinkle with parsley.