Yep, the wrapper works great, you have to modify the RoundCube code, but;
It might be easier to rewrite the MySQL access in the Joomla portion that implies login, to access the roundcube database.
Also you will have to modify cookies and cookie varibles in both Joomla and RoundCube.
These steps might improve seemless integration.
Other steps, such as; modifying the admin panel functions in joomla to automatically create an email account in cpanel, will avoid the hassel of having two admin areas.
RoundCube can be set up to automatically create the entry in the RoundCube database on first loggon.
No manual intervention needed then!
Other functions of email account administration should also be developped, ie: autoresponder, boxtrapper, and email forwarding...
Good Luck!