You just have to create a "prefix" and create elements with it.
Here is a sample code creating what you want and displaying it to the console.

Here is the code:

try {

Namespace prefix = Namespace.getNamespace("xsl", "http://www.w3.org/1999/XSL/Transform");

Element elt = new Element("stylesheet", prefix).setAttribute("version","1.0");

elt.addContent(new Element("output",prefix).setAttribute("method","html").setAttribute("indent","no"));

elt.addContent(new Element("template",prefix).setAttribute("match","/html"));

XMLOutputter outputter = new XMLOutputter(" ", true, "ISO-8859-1");

outputter.output(elt, System.out);

And here is the result: