'extending' the xhtml schema

Hello-
I'm trying to extend the xhtml schema by adding my own attribute.  I imported the xhtml schema, and used <xs:any namespace="http://www.w3.org/1999/xhtml"/> to mix xhtml with my own elements, which works fine.  But, I want to be able to add one of my own attributes to any element in the xhtml schema, which will be processed by my own processor later.
Something like:

<myns:mynode xmlns:myns="http://my.schema.com/"

 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
   <myns:a>
     <myns:b><xhtml:div class="someClass" myns:myatt="this is my attribute">This is div content</xhtml:div></myns:b>
   </myns:a>
</myns:mynode>

Is this possible?
I've tried using <xs:redefine>, but it seems that is only for extending types and groups, etc.

Thanks