You can create a Flat OPC XML file containing your DrawingML.
Then you add it to your docx open in Word. You can do that using VBA (a macro), or VSTO (a Word Add-In).
For more on Flat OPC XML, see http://blogs.msdn.com/b/ericwhite/archive/2008/09/29/the-flat-opc-format.aspx
You can add the Flat OPC XML using Range.InsertXml. See also Can I use VSTO instead of Open XML to manipulate altChunk features?
VSTO is a .NET thing. Generally you'd code in C# or VB.NET. You can use Microsoft's Open XML SDK as necessary. So Java doesn't usually have a role. Having said that, I do know of companies that are committed to Java server side and using docx4j there, and want to use the same technology client side, and so use docx4j IKVM'd as a DLL. In this context, that could be how you create your DrawingML (ie using JAXB), then produce the Flat OPC XML.