<p>Anyone know off the top of their heads how to convert a System.Xml.XmlNode to System.Xml.Linq.XNode?</p>

I don't think there is, but why would you need to? Each is the lowest 'leaf' of the Xml structure for different ways of reading the document.
If you use Linq to Xml and XDocument you'll have all the linq-style syntax and new functionality, but really all that's about selecting a node.
Once you have the element that you're dealing with, why do you need to switch?