In a small mixing bowl, mash one can of beans until nearly smooth. In a 4- to 6-quart slow cooker, combine the mashed beans, whole beans, and the next 13 ingredients (through black pepper). Cover and cook on low 6 to 8 hours or on high 3 to 4 hours. Remove ham hock. If desired, cut meat from bone and return meat to soup; discard bone. Top with yogurt and sprinkle with green onions. Serve with lime wedges for squeezing.