Hi ;)
I think you will be work in:
--Client side
=include/client/open.inc.php (here you will add the field)
=open.php... (here to gett the new information)
=Add a new colum in your bd
staff side:
=include/staff/ticket-view.inc (to show to the staff the new information)
=Include/class.ticket
mmm I think it´s all, but i´m not sure about it