Please search the forums, there are multiple posts about how to do this.
This article may also assist you.
How to add a DB populated drop down box to your tickets: 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)