External user database?

[geoB]

I'm experimenting with roundcube on a system that runs a Symfony application.  That application is behind a Symfony firewall and uses FOSUserBundle to authenticate against a MySQL database. Is there any means to use that database to logon existing application users.  Roundcube would not add users, i.e. 

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$config['auto_create_user'] = false;.

It looks like I need to add a MySQL database for postfix/dovecot.  Once that's in place I can add an event listener to the Symfony application to update the postfix/dovecot database plus the roundcube database with new user data.  Any thoughts?