Trim fat from steak. Place steak in a 4 1/2-quart electric slow cooker coated with cooking spray. Top with pepper stir-fry. Combine tomato sauce and steak sauce; pour over peppers. Cover and cook on high heat setting 1 hour. Reduce heat to low setting, and cook 7 hours.
Remove steak from slow cooker with a slotted spoon. Shred steak with 2 forks. Return shredded steak to slow cooker; turn off slow cooker. Add salsa, sugar, and salt. Cover and let stand 15 minutes.