I believe that you'd need a custom add-on to do this, because you'd have to have a complete overhaul of some core systems in XenForo to make this work. On top of that, I'm not entirely sure it'd help at all – I've never, ever known anyone on XenForo need or want to touch the URL structure to help their site rank in Google. The default URLs are plenty good enough for SEO purposes, and the content will always be the far more important factor.
Changing the URL structure is a micro-optimisation at best – and I would personally be worried about implementing this personally, as doing so may even hurt your search rankings if there are any bugs at all in the method used to do it (if it causes errors or sends users to the wrong place, for example).