lucy24 - 7:58 am on Apr 12, 2013 (gmt 0)
on the same server
I got the impression this is shared hosting. So it would have to be the same userspace, right? But the OP makes it sound as if the sites all live in the same place, so this should not be impossible.
This is all assuming you have full control of your htaccess file and can write any rules you need. You will need to proceed very carefully, because your CMS has already made an htaccess. You have to make sure not to conflict with any of its actions.
Start by explaining what the physical arrangement is. Do all of your domains live side by side in one userspace? Or do you have a primary domain, with "add-ons" in separate directories inside your first directory?