Hi there, the underlying approach of interest here is "altChunk" of type docx.
If you include your docx as an altChunk, then the first time it is opened in Word, Word will incorporate its contents.
If you need that to happen earlier (eg to create HTML or PDF output using docx4j, or for greater control/certainty, or because your users might open the docx in something other than Word), then Plutext's Docx4j Enterprise Edition's MergeDocx component can do this.
By the way, docx4j's OpenDoPE implementation now also supports binding an XML element which contains base64 encoded docx content. It'll convert that to an altChunk, and if you have the MergeDocx component, that'll in turn convert it to "real" docx content.
You can download a trial of the Enterprise Edition, from http://www.plutext.com/m/index.php/products