The BPMNXmlConverter class has a map of converters it uses:
<code>
public static void addConverter(String elementName, Class<? extends BaseElement> elementClass,
Class<? extends BaseBpmnXMLConverter> converter) {
convertersToBpmnMap.put(elementName, converter);
convertersToXMLMap.put(elementClass, converter);
}
</code>
Simply overwrite the user task default one with your own implementation