You can use a java-class, which you add to the spring-context (for example) and make spring call some kind of init-method. The init-method should call the static addConverter(…) method on org.activiti.bpmn.converter.BpmnXMLConverter, which can override the current converter for UserTaskXMLConverter.getXMLType().
This way, when your app is loaded and spring-context ready, the converter will be rigged with the custom converter.