Integrating single sourced XML-based documents from a CMS to a Knowledge base

There is no "out of the box" solution that I'm aware of, but Lithium has a very robust API that allows you to "push" data into the community, as well as a great customization platform that alllows you to "pull" data into the community from other sources.  You could either make a Lithium REST API call to push data into Lithium from your CMS system, or you could sign up for our http.client beta program and creat an Endpoint in Studio that pulls data in from your CMS system (assuming it has an API that will let you do so).