Hi Gauravk
What you want is actually possible, for example with a custom direct login page, see:
/http://www.dropdeaddick.com/files/cpanel-webmail-login.php
cPanel Webmail Direct Login Script
change following two lines (comment out squirrel-mail, uncomment roundcube):
# $mail_app = '/3rdparty/squirrelmail/'; // Read Mail Using SquirrelMail
$mail_app = '/3rdparty/roundcube/'; // Read Mail Using RoundCube
If you have some html skills you can also add this to a custom cPanel login theme for all your clients, see documentation for more info:
How to Create Custom-Branded Login Pages
the important line is the following hidden form-field:
<input type="hidden" name="goto_uri" value="/3rdparty/roundcube/" />
downside:
users wont have access to the cPanel features like redirects, spam-settings etc.
We have written a small roundcube addon which adds a link to the default cpanel page in roundcubes settings tab, however this will be deleted on each roundcube update and i did not have time to solve this yet, so maybe add a feature request for this too?