You need to add your custom fields ORM code to the values: https://github.com/osTicket/osTicket/blob/develop/include/client/tickets.inc.php#L129
Add the header just like here: https://github.com/osTicket/osTicket/blob/develop/include/client/tickets.inc.php#L212
Add the cell just like here: https://github.com/osTicket/osTicket/blob/develop/include/client/tickets.inc.php#L249
Make sorting work by adding your translation here: https://github.com/osTicket/osTicket/blob/develop/include/client/tickets.inc.php#L40