When creating a new XWPFDocument from scratch, there is no way to create an OpenXml footnotes document part. I am submitting a patch that adds an XWPFFootnotes class and also makes the existing XWPFFootnote object implement the IBody interface so that it can receive paragraph and table elements. The same thing may need to be done for endnotes in the future.
Created attachment 27271 [details] Text diff
Created attachment 27272 [details] Binary tar.gz of the added files. Adding two files: XWPFFootnotes.java and TestXWPFFootnotes.java.
Created attachment 27273 [details] Text diff Initially forgot to add TestXWPFFootnotes.java to the project.
Thanks for this, patch applied in r1144308.