Cut 2 oranges into 6 wedges each. Thread lamb and oranges alternately onto 4 skewers. Thread onions onto separate skewer. Squeeze juice from 1 of the remaining oranges into small bowl.
Add barbecue sauce and allspice; mix well. Reserve half the barbecue sauce mixture.
Grill skewers 15 min. or until lamb is done, turning and brushing with remaining barbecue sauce mixture for the last 5 min.
Remove onions from skewer; place in large bowl. Peel and section remaining oranges.
Add to onions with arugula and dressing; mix lightly.