USE DOM
for example
-------------------------------------------------------------
var m_xmlDom = new ActiveXObject("Microsoft.XMLDOM");
m_xmlDom.load("a.xml");//载入a.xml文件
<!--加入XML结点开始-->
var newDom = new ActiveXObject("Microsoft.XMLDOM");
newDom.loadXML("<newNode>.....</newNode>");//这儿是需要加入的xml节点
var newElem = newDom.documentElement;
m_xmlDom.documentElement.insertBefore(newElem,m_xmlDom.documentElement.childNodes.item(0));
<!--加入XML结点结束-->