Hi,
DocumentAssembler was not designed to take as input any arbitrary XML. There are many forms of XML that are not appropriate for it. Instead, for this version of DocumentAssembler, my recommended approach is that you reformulate your XML so that your data is in the form that is accessible by DocumentAssembler.
The key benefit of DocumentAssembler is not in its general purpose way of accessing XML, but is instead, in the way that you can take data from compatible XML and combine to formulate a valid, correctly formatted DOCX. Attempting to do more than this was beyond the scope of the project.
-Eric