PREHEAT oven to 350°F. Place sweet potatoes on microwaveable plate. Microwave on HIGH 8 min. Turn potatoes over; continue microwaving 9 to 10 min. or until very tender. Let stand 5 min. Cut potatoes in half lengthwise. Scoop pulp into large bowl; discard skins.
BEAT sweet potatoes, margarine, cinnamon, ginger and nutmeg in medium bowl with electric mixer on medium speed until well blended. Spoon into lightly greased 9-inch square baking dish; top with marshmallows.
BAKE 15 to 20 min. or until potato mixture is heated through and marshmallows are lightly browned.