PREHEAT grill or grill pan over medium-high heat.
CUT ham into 1" wide strips and fold into quarters.
PLACE ham, red onion, green bell pepper and pineapple chunks onto skewers. Brush on all sides with olive oil.
COMBINE 1/2 cup reserved pineapple juice, brown sugar, ketchup, Worcestershire sauce, vinegar, salt, pepper, and garlic powder in a small saucepan. Bring to a boil for 3-5 minutes. Set aside.
GRILL skewers, turning frequently and brushing with sauce during last half of cooking,until vegetables begin to soften and kebabs begin to char, about 5-7 minutes.
REMOVE skewers from grill, garnish with sliced scallions, and serve with extra barbecuesauce.