Mixing my vocabularies

Hi

Can I inter-mix other vocabularies with an XML Schema defined vocabulary in an instance document, and still have a valid document? If so, how? 

N.B. the answer should ideally not involve any changes to the XML schema, just to the conformant document (e.g. I cannot use <xsd:anyAttribute namespace="##other"/>, etc).

This is the sort of thing I have in mind:

<message xmlns="http://www.mynamespace.com" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.mynamespace.com my_schema_location" xmlns:fred="http://fred">
<my_valid_element fred:attribute_from_another_vocabulary="hello">...</my_valid_element>
</message>

I assume that the above would fail validation.

Thanks for any help.

Mark Seaborne