Customizable? Sure.
Is it customizable with in the Admin Panel ui? No.
You will have to go down to the system level and edit the source files, and requires that you also edit the MySQL tables. It's rather involved but not particularly difficult if you know html, and php.
This article ought to get you started and pointed into the right direction:
http://www.tmib.net/content/osticket-16-how-add-db-populated-drop-down-and-admin-interface-version-10(http://www.tmib.net/content/osticket-16-how-add-db-populated-drop-down-and-admin-interface-version-10)