Line the inside of a 9x9x2-inch baking pan with foil; grease foil. Cover the outside of the pan with foil. Place butter in pan and tilt pan to coat. Stir in brown sugar until moistened; spread evenly. Arrange peaches over the brown sugar mixture; top evenly with the 1/2 cup berries. Set aside.
Prepare cake mix according to package directions, except use only 1/3 cup water. Stir pecans into cake batter. Spoon batter over peach-berry mixture; spread evenly.
Prepare grill for indirect grilling, except omit drip pan. Test for medium heat above the center of grill. Place baking pan on the grill rack in the center of the grill (not over heat). Cover and grill about 30 minutes or until a toothpick inserted near the center of the cake comes out clean.
Cool in pan for 10 minutes. Invert cake onto a serving platter; carefully peel off foil. Let stand for 20 minutes; serve warm. If desired, top with additional fresh berries. Makes 9 servings.