Add the beans, sweet potato, onion, tomatoes, stock, cumin, salt, and chili powder to the slow cooker. Cover and cook on LOW for 7-8 hours or HIGH for 4-5 hours.
Puree soup with an immersion blender or a regular blender, working carefully in batches.
Stir in the the rice, using 1-2 cups depending on how thick you'd like the soup.
Let sit, covered, for at least 5 minutes before serving to ensure the rice is warm.
Season to taste with additional salt if needed. Serve with shredded cheese, sour cream, and/or avocado, as desired.