In 3 1/2- to 4-quart slow cooker, layer onion, squash and apples. In small bowl, mix syrup, juice concentrate, salt and pepper; pour over apples.
Cover; cook on Low heat setting 5 to 6 hours.
Gently stir in sausage. Increase heat setting to High. Cover; cook 20 to 30 minutes or until sausage is thoroughly heated.