假设你有原始文档:

 var doc = XDocument.Load(...);

然后创建一个新元素(不是文档)

then create a new element (not a document)

  //XDocument doc = new XDocument(      
  //  new XElement("stock",  /* how to go inside existing "stock"? */
   var newElement =  new XElement("items", 
                      new XElement("productname", "Toothpaste"),
                      new XElement("brandname", "CloseUp"),
                      new XElement("quantity","16"),
                      new XElement("price","15"));

然后插入:

  doc.Element("stock").Add(newElement);

  doc.Save(....);