How to define the content of an xs:any element

[Erik]

Hi
I'm writing a schema for a particular soap message. In the soap schema
any content is allowed in the header and body elements. Is it possible
to define the elements allowed in the header and body section in my
own schema but still be able to create xml files that will validate
against both the soap schema and my schema? E.g., can I somehow
specify that one specific element is mandatory and another is optional
in the header section?

Thanks, Erik.