It is possible to add this new order by but you need to do code changes in a lot of places . If you are not skilled at coding you should hire someone since a bad code added on this area can have major impact.
You have to add new order by way
should by ‘order_by’ =>’modified’
For ex in property_list.php you have this switch/ case code http://prntscr.com/gl1sff .
You should add a new branch that look like
   case 10:
                    $meta_order=”;
                    $meta_directions=’DESC’;
                    $order_by=’modified’;
                    break;