Single RoundCube setup for multiple ISPConfig3 servers

[Hans]

Since í'm using ISPConfig3, i'm interested in a single installation of the RoundCube webmail client, for all my ISPConfig3 servers.
For maintenance reasons, i prefer one single setup instead of one RoundCube webmail client on every ISPConfig3 server.

Within the RoundCube configuration file main.inc.php i can change the line $rcmail_config['default_host'] = 'localhost'; into $rcmail_config['default_host'] = '';
By doing this an extra field on the login screen appears, so the hostname of the imap-server can be filled in there.
However this is not what i want.
Another possibility is to give one ISPConfig 3 server the task of mailserver and nothing else.
By doing this, RoundCube needs only one host, so that's a possibility, but it's not what i prefer.

So my question is: does anybody know how to configure RoundCube, so that it can access multiple hosts?