....
Element root = DocumentHelper.createElement("vevent");
root.addNamespace("", "urn:ietf:params:xml:ns:xcal");
XmlHelper.toXml(root, "class", event.getClassification());
...

System.out.println(element.getNamespaceURI()); // пусто
System.out.println(element.getNamespace()); 
// вывод org.dom4j.Namespace@babe [Namespace: prefix  mapped to URI ""]

XmlHelper.toXml(root, "class", event.getClassification());

public class XmlHelper {

	public static <T> void toXml(Element e, String en, T value)
	{
		if (e == null || en == null)
			throw new IllegalArgumentException();

		if (value != null) {
			e.addElement(en).addText(value.toString());
		}
	}

e.addElement(en)

Element root = DocumentHelper.createElement(new QName("vevent", new Namespace("", "urn:ietf:params:xml:ns:xcal")));