Adding more menu in the menu settings? In Drupal 7, in admin/structure/menu/settings we have the main links and the secondary links. 

Can we add third one here? Is there any way to get the list of all menus created in backend?
Can we add third one here?


Yes, we can. Use hook_form_alter(), alter form_id menu_configure.

How do I know what form_id? Let's see menu_menu() at http://api.drupal.org/api/drupal/modules--menu--menu.module/function/menu_menu/7


  Is there any way to get the list of
  all menus created in backend?


Again, We can. Select menus in table {menu_links} where {menu_links}.menu_name = management.


    $result = db_select('menu_links', 'm')
    ->fields('m')
    ->condition('menu_name', 'management','=')
    ->execute()
    ->fetchAssoc();