In case you are using latest osTicket (1.9.2) or able to upgrade to, you could use LDAP or HTTP/Webserver authentication for login into osTicket. Can you please provide more info about your setup (like: http://forum.osticket.com/d/discussion//please-read-before-requesting-assistance#latest), what kind of "existing user DB" you have (mysql database, ldap, ...?) and if authentication against your webserver or ldap solve this problem?