If you use frozen chicken, make sure it is completely defrosted. If you cook it frozen, it will indeed cook but it will make a very “wet” sauce. Place your chicken in the bottom of the crock pot. Top with onions, then pineapple. Pour the entire bottle of BBQ sauce over the whole thing. Note: If you want a sweeter BBQ, add up to half a cup of the pineapple juice you drained, depending on how sweet you want it. This is optional. Turn your crock pot on LOW and cook for 8–10 hours (or on HIGH for 4–6). An hour or two before it is finished cooking, carefully remove the chicken from the crock pot and shred it using two forks. Watch out, it will be hot! Place chicken back in the crock pot and stir well to combine. Finish cooking an additional hour or two so the chicken can absorb most of the sauce. Serve on hamburger buns and top with coleslaw if desired.