Preheat oven to 350°.
Combine first 5 ingredients in a large bowl; cut in margarine with a pastry blender or 2 knives until mixture resembles coarse meal. Combine maple syrup, maple flavoring, egg whites, and yogurt; add to dry ingredients, stirring just until dry ingredients are moistened.
Divide batter evenly among 18 muffin cups coated with cooking spray; sprinkle walnuts evenly over batter. Bake at 350° for 25 minutes or until muffins spring back when touched lightly in center. Remove from pans immediately; let cool on a wire rack.