Combine first 7 ingredients in a bowl, and stir well. Spoon sweet potato mixture into an 8-inch square baking dish coated with cooking spray.
Combine 1/2 cup brown sugar and flour in a bowl, and cut in 2 tablespoons chilled margarine with a pastry blender or 2 knives until the mixture resembles coarse meal. Stir in the chopped pecans, and sprinkle over sweet potato mixture. Bake at 350° for 30 minutes.