Lightly coat a 3 1/2- or 4-quart slow cooker with cooking spray; set aside.
In cooker combine frozen berries, pie filling, and sugar.
Cover and cook on low-heat setting for 3 hours. Turn cooker to high-heat setting. In a medium bowl combine muffin mix, the water, and oil; stir just until combined. Spoon muffin mixture over berry mixture.
Cover and cook for 1 hour more or until a wooden toothpick inserted into center of muffin mixture comes out clean. Turn off cooker. If possible, remove crockery liner from cooker. Cool, uncovered, for 30 to 45 minutes on wire rack before serving.