Melt margarine in large skillet over medium heat. Add onion; cook until crisp-tender, stirring occasionally.
In 3 1/2- to 4-quart slow cooker, combine onion and all remaining ingredients except cream cheese; mix well.
Cover; cook on low setting for 6 to 8 hours.
About 40 minutes before serving, place about 1/3 of mixture at a time into blender container or food processor bowl with metal blade. Cover; blend or process on high speed until smooth. Return mixture to slow cooker.
Stir in cream cheese. Cover; cook on low setting an additional 25 to 30 minutes or until cheese is melted, stirring with wire whisk until smooth.