take a look at \support\include\class.ticket.php
circa line 1456ish
should look like:
if($var && !in_array(strtolower($var),array('email','phone','other')))
$errors='Invalid source - '.Format:($var);
change it to
if($var && !in_array(strtolower($var),array('email','phone','other','fax')))
$errors='Invalid source - '.Format:($var);
Ok there is more to do... the database table structure itself needs to be changed. You will need to run a mysql cli or gui and execute the following command: alter table ost_ticket change source source ENUM('Web','Email','Phone','Fax','Other');
You should probably make sure that they are in the same order every where (which I didn't do but you should).