How to group all custom modules under one menu using hook_menu?
Thanks to kalabro for the answer. For those who are new to module development and need the same functionality, I've posted the code below :

  function hook_menu() {
  $items['admin/settings/custom'] = array();
  $items['admin/settings/custom/module-name'] = array(
  //other code here
  ); 
}


You can use the "$items['admin/settings/custom'] = array();" in a separate module also. 

Note : 
Be sure to flush the cache. Sometimes you might need to disable and then enable the modules again for this to work...