In large bowl, place rye berries; add enough water to cover. Let soak in refrigerator at least 8 hours but no longer than 24 hours; drain.
Spray 3- to 4-quart slow cooker with cooking spray. Place drained rye berries and remaining ingredients except squash and walnuts in slow cooker. Stir to mix.
Cover; cook on Low heat setting 7 to 8 hours or until rye berries are chewy but tender.
Stir in squash. Increase heat setting to High. Cover; cook about 30 minutes or until squash is tender. Stir in walnuts.