1) In scp/tickets.php - line 314 add the source to the array
if($_POST && !in_array($_POST,array('email','phone','other','inperson')))
2) In include/staff/newticket.inc.php - line 44+ add the option
<option value="inperson" <?=($info=='inperson')?'selected':''?>>In Person</option>
3) In the database - table ost_ticket - need to edit the Datatype for column source to: ENUM('Web','Email','Phone','Inperson')
Ensure that in all three steps the value is the same, in this case inperson
Good luck.