Took a little look at this. Perhaps you can alter the function createTicket in class.pop3.php and add the following line:
This gets the date from the email headerinfo, but not in the right format as far as I know.
Off course the second thing you should alter is the function create() in the file class.ticket.php. It now says something like "INSERT INTO '.TICKET_TABLE.' SET created=NOW() '."
The NOW() part should then be replaced with your emaildate, but only when available (since the create function is also used for non email tickets).
Anyway, it's not the easiest thing to do but perhaps I have given you a headstart.