In a medium bowl, combine the shredded chicken and Hawaiian BBQ sauce. Mix well to coat the chicken evenly. Heat the mixture in a saucepan over medium heat until warmed through, about 5-7 minutes.
While the chicken is heating, prepare the coleslaw. In a separate bowl, mix together the coleslaw mix, mayonnaise, apple cider vinegar, sugar, salt, and black pepper until well combined.
Toast the hamburger buns lightly on a skillet or in a toaster.
To assemble the sandwiches, place a generous portion of the BBQ chicken on the bottom half of each bun. Top with a scoop of coleslaw and sliced green onions. If desired, add sliced pineapple for extra flavor.
Place the top half of the bun on each sandwich and serve immediately.