Mix ketchup, molasses, water and seasoning mix in 4-quart slow cooker. Add chicken to slow cooker; stir to coat.
Cook 2 hours on High or until chicken is cooked through. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C). Remove chicken from slow cooker.
Shred chicken using 2 forks. Return chicken to slow cooker. Stir in cream cheese and shredded cheese. Sprinkle with onion before serving.
