Place 15-bean soup mix in a large bowl and cover with several inches of cool water; soak in refrigerator 8 hours to overnight. Drain and rinse.
Place 15-bean soup mix, ham bone, cooked ham, chicken broth, water, onion, carrots, great Northern beans, garlic, black pepper, salt, and bay leaf in a slow cooker; stir to combine. Cook on Low for 8 to 10 hours.
