Heat oven to 375°F. In large bowl, mix sugar and flour. Stir in frozen berries until coated. Spread in ungreased 8-inch square (2-quart) glass baking dish.
Bake uncovered 25 minutes.
Meanwhile, place cereal in resealable food-storage plastic bag; seal bag and slightly crush with rolling pin or meat mallet (or slightly crush in food processor). In medium bowl, mix cereal, walnuts, brown sugar and butter until blended.
Stir fruit well. Sprinkle cereal mixture evenly over top. Bake 20 to 25 minutes longer or until light golden brown and bubbly. Let stand 5 to 10 minutes before serving. Serve with ice cream.