var BpmnModdle = require('bpmn-moddle');

var moddle = new BpmnModdle();

var xmlStr =
  '<?xml version="1.0" encoding="UTF-8"?>' +
  '<bpmn2:definitions xmlns:bpmn2="http://www.omg.org/spec/BPMN/20100524/MODEL" id="empty-definitions" targetNamespace="http://bpmn.io/schema/bpmn">' +
  '</bpmn2:definitions>';


moddle.fromXML(xmlStr, function(err, definitions) {

  // update id attribute
  definitions.attr('id', 'NEW ID');

  // add a root element
  var bpmnProcess = moddle.create('bpmn:Process', { id: 'MyProcess_1' });
  definitions.get('rootElements').push(bpmnProcess);

  moddle.toXML(definitions, function(err, xmlStrUpdated) {

    // xmlStrUpdated contains new id and the added process

  });

});