Quite simple, actually (even with DOM standards):
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
factory.setNamespaceAware(true);
DocumentBuilder builder = factory.newDocumentBuilder();
Document d1 = builder.parse(...);
Document d2 = builder.parse(...);
d1.getDocumentElement().appendChild(d1.importNode(d2.getDocumentElement(), true));