I installed Roundcube on my server (Debian Wheezy Apache) now and it's working fine. Now a question came up: Is it possible to change on the fly back to SquirrelMail and vice versa? I do believe that there'll be some user liking one or another and it would be great if the Mail Client could easy be changed. Thanks.
To answer myself, I'd posted at Feature Request: http://www.howtoforge.com/forums/showthread.php?t=63517 Included the solution how to use both Mail Clients on the same Server Server wide.