Namespace question

Is there a way to tell a stylesheet to process an instance
in namespace X, as though it were really in namespace Y?

E.g., suppose I get a boatload of content from a customer
marked up in a minor variant of docbook identified with a
different namespace (e.g., xmlns="http://myschema.org/").
Even though that's not the best way to identify a DocBook
variant, let's assume I can't edit their source, and I
don't want to edit the standard stylesheets.

Instead, I'd like to do something in my customization layer
that effectively tells the imported stylesheets to treat
input in the "http://myschema.org/" namespace as though it
were in the docbook namespace.

Anyone have any suggestions?

Thanks,
Dick Hamilton