Toss shrimp, plums, ginger, olive oil, salt, and pepper in a large bowl. Thread 3 shrimp and 3 plum sections alternately onto each of 4 (10-inch) skewers.
Spoon 1/2 cup Sweet Hoisin Sauce into a cup; place remaining sauce in a serving bowl. Place kebabs on a grill rack coated with cooking spray; cook for 4 minutes. Turn kebabs, and brush with about 1/4 cup sauce. Cook 4 minutes. Turn the kebabs; brush with about 1/4 cup sauce. Cook 2 minutes, turning once. Serve remaining 1/2 cup sauce as a dipping sauce. Garnish with onions and lime wedges, if desired.
Totals include Sweet Hoisin Sauce.