In a way, Xenforo is already a CMS - with huge expansions on this through addons....
You have the ability to create pages with custom HTML/php callbacks (essentially giving php capability), and with the nodes as tabs add-on, these pages can become higher level pages, and in a way, fully fledged pages. The only problem is that it get's trickier to give permissions to edit/create pages to non administrators...
Mind you, if you're looking for a legitimate hands-down, professional CMS, go with MODx. Just simply copy all the CSS/HTML markup from your forum (while logged out, and make sure to change the URLs to the CSS pages and any other resources), paste it into MODx's template system, strip out all the content except for the generic layout HTML/css, then put the appropriate MODx tag there... voila! Dedicated CMS!!!