As with Windows user database support, you can use ACS ODBC-compliant relational database support to use existing user records in an external ODBC-compliant relational database. Configuring ACS to authenticate against an ODBC-compliant relational database does not affect the configuration of the relational database. You can use the Windows ODBC feature to create a data source name (DSN), which specifies the database and other important parameters that are necessary for communicating with the database. Among the parameters that you provide are the username and password that are required for the ODBC driver to gain access to your ODBC-compliant relational database.
Take a look at this link.
http://www.cisco.com/en/US/docs/net_mgmt/cisco_secure_access_control_server_for_windows/4.2/user/guide/UsrDb.html#wp462609
