There has been a mod for adding an agency drop down since 1.7ST was launched in the mod section.
http://www.osticket.com/forums/showthread.php?t=13927(http://www.osticket.com/forums/showthread.php?t=13927)
which will refer you to the article at:
http://www.tmib.net/agencies-dropdown-and-admin-ui-osticket-17st(http://www.tmib.net/agencies-dropdown-and-admin-ui-osticket-17st)
Then you will just need to figure out how to add Agencies to the osTicket dashboard or the Mod: Reports (also in the Mods section)