Upload a short sample docx to the webapp linked in the menu above; then click through document.xml
You'll see the w:numPr XML you need in w:pPr
Click on your w:p or the w:pPr to generate code. It'll be similar to line 339 of https://github.com/plutext/docx4j-Impor ... elper.java
The numbering stuff points to an entry in the numbering part. So you'll need a corresponding entry there (either pre-existing, or one you add).
You can also click through to relevant OpenXML documentation via the webapp.