I think you are misunderstanding how custom fields are supposed to work. Name and Value map directly to the meta_key and meta_value columns in the wp_postmeta table, so it would be impossible to add a third column as you describe without altering the table definition. If you want to add a link to a post as a custom field, the Name should be ‘link’ (or something similar), and the Value would be the link itself. You can add as many custom fields as you want.