There are no established standard format to allow interoperability between different wiki engines. This is not the main objective of this project but as it is something highly desirable this page list some efforts being made in this direction.
Martin Junghans, Dirk Riehle and Umit Yalcinalp published the article "An XML Interchange Format for Wiki Creole 1.0" available here.
Also, MediaWiki has a extension called XML Bridge that parses their wiki syntax to XML. But apparently there is no effort to use a general XML representation (they call their XML representation mwxml).