Cook sausage in 10-inch skillet over medium-high heat 9 to 11 minutes, stirring occasionally, until no longer pink; drain.
Spray 3- to 4-quart slow cooker with cooking spray. Mix sausage, stir-fry vegetables, pepperoni, pizza sauce and Italian seasoning in cooker.
Cover and cook on Low heat setting 4 to 6 hours.
Stir in tomatoes. To serve, fill buns with sausage mixture and sprinkle with cheese.