I'll add it when I get a chance in the next day or so.
Or you could add it, and send me a patch
You need to define its content type in org.docx4j.openpackaging.contenttype.ContentTypes, and then make a one line addition to org.docx4j.openpackaging.contenttype.ContentTypeManager.createPackage()
Some or all of "potm", "potx", "ppam", "ppsm", "pptm" also need the same treatment.
cheers .. Jason