WordprocessingMLPackage wordMLPackage = WordprocessingMLPackage.createPackage(PageSizePaper.LETTER, false);

XHTMLImporterImpl XHTMLImporter = new XHTMLImporterImpl(wordMLPackage);
XHTMLImporter.setRunFormatting(FormattingOption.CLASS_PLUS_OTHER);
NumberingDefinitionsPart ndp = new NumberingDefinitionsPart();

wordMLPackage.getMainDocumentPart().addTargetPart(ndp);
ndp.unmarshalDefaultNumbering();

wordMLPackage.getMainDocumentPart().getContent().addAll(XHTMLImporter.convert({STRING_OF_HTML_CONTENT}, null));

File exportFile = new File(somepath/sample.docx);