Hi - Yes, it is pretty much possible to do anything. However, just because you CAN doesn't mean you SHOULD.
For example, my site uses a YJ template, and I run WHMCS. There are ways to share login data. In reality, since no one uses the Joomla! portion of the site for posting, I don't sync users. In fact, since WHMCS handles billing and support tickets, I don't even allow new users to register on the Joomla! part of the site as I can't think of a good reason to do so.