secure acs & external database

"If an authentication attempt fails against its internal list of users, the Cisco Secure ACS will try the selected databases configured in the Unknown User Policy. The external databases are attempted sequentially, in the configured order. Upon a successful attempt, the user is added to the Cisco Secure ACS internal database but marked for authentication by the appropriate database. For subsequent authentication attempts, ACS will try the supplied credentials directly against the previously successful external database."

How can I get the users in my exchange 2000 server to be put into the external database of my cisco secure ACS server? (so that I can check for authentication) I have been trying to solve these problem for weeks now!! Any help is greatly appreciated!!!!!!!!!
-Yasemin