In medium bowl, combine marinara sauce and tomatoes with their juice.
Spray 4½- to 6-quart slow-cooker bowl with nonstick cooking spray.
Spoon 1 cup tomato-sauce mixture into bowl.
Arrange one-fourth of noodles over sauce, overlapping noodles and breaking into large pieces to cover as much sauce as possible.
Spoon about ¾ cup sauce over noodles, then top with one-third of ricotta (generous ½ cup) and ½ cup shredded cheese.
Spread half of spinach over cheese.
Repeat layering, beginning with noodles and using frozen crumbles instead of spinach; repeat layering using remaining spinach.
Place remaining noodles over spinach, then top with remaining sauce and shredded cheese.
Cover slow cooker with lid and cook as manufacturer directs on Low for 2½ to 3 hours or on High for 1½ to 1¾ hours or until noodles are very tender.