import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;
import javax.xml.transform.TransformerFactory;
Document
doc = ... // your existing DOM document
DOMSource source = new DOMSource(doc);
StringWriter xmlAsWriter = new StringWriter();
StreamResult result = new StreamResult(xmlAsWriter);
TransformerFactory.newInstance().newTransformer().transform(source, result);
StringReader xmlReader = new StringReader(xmlAsWriter.toString());
InputSource viola = new InputSource(xmlReader);