Mix sugar, paprika, mustard, cumin, garlic powder, onion powder, cayenne, salt, and pepper together in a small bowl. Rub the mixture evenly over chicken pieces.
Transfer chicken to the slow cooker. Pour barbecue sauce over the chicken and toss to coat.
Cover and cook until chicken is tender, 4-6 hours on low.
OPTIONAL: For crispy skin, transfer chicken to a foil-lined baking pan. Brush additional barbecue sauce over the chicken and broil approximately 5 minutes per side, until lightly charred and crisp.