Hi,
I believe it is possible without any major issues. I do not install Rounbcube but I think the actions should be:
1. Install Rounbcube.
2. Configure Rounbcube.
3. Configure a new VirtualHost in Apache.
4. Disable the Webmail module.
5. Enable the Apache service.
6. Test the new integration
NOTE: Keep in mind that Zentyal installs and uses Mysql service, so, in case you need to use this local database service, just remember that the root password is located at '/var/lib/zentyal/conf/zentyal-mysql.password'.
--
“This world is ours, and by the Holy Light we will keep it safe, now and forever".