Upcast & Downcast
The DocBook XSL project (http://docbook.sf.net/) has exactly what you
want - an XSL stylesheet that converts DocBook into WordML. The
project also happens to have the reverse as well; XSL stylesheets that
convert WordML into DocBook.
You may need to look in the CVS repository to find the stylesheet(s);
I'm currently working towards having them included in the distribution.