Spray inside of 4-quart slow cooker with cooking spray. Sprinkle roast pieces with seasoning mix; place in bottom of slow cooker. Pour sauce over meat. Cover; cook on LOW 9 to 11 hours or until beef is tender.
Turn slow cooker to HIGH. Remove beef with slotted spoon to work surface. Pull into shreds with 2 forks or chop fine. Return to slow cooker. Add beans and stir to mix with sauce. Cook 5 minutes or until thoroughly hot.
Spoon about 1/2 cup beef mixture down center of each tortilla. Top with cheese and lettuce. Fold up bottom; fold in sides and roll to wrap. Secure with wooden pick, if desired.