Maybe it\'s me, but it appears that if you have an XSD
You should be able to extend your schema with the
element for extensibility - see W3Schools for details.
When you add the processContents="lax"
then the .NET XML validation should succeed on it.
See MSDN docs on xs:any for more details.
Update: if you require more flexibility and less stringent validation, you might want to look at other methods of defining schemas for your XML - something like RelaxNG. XML Schema is - on purpose - rather strict about its rules, so maybe that's just the wrong tool for this job at hand.