I have solved it now on my side.
If you want to check the plugin, please goto
http://mail4us.net/dev and register a test account.
There is a global embeded nabble mailing list at the login screen (see button Mailing List). When a user is in authenticated state he is logged into nabble automatically (see settings in "Settings->Server Settings"). Also the user has the choice to be redirected to his own nabble application (will be improved later to have more than only one choice).