XmlNode root = doc.ReadNode(reader);
                XmlElement elem = doc.CreateElement("test_node");
                elem.InnerText = "test_text";

                root.AppendChild(elem);

                XmlWriterSettings settings = new XmlWriterSettings();
                settings.Indent = true;
                XmlWriter writer = XmlWriter.Create(@"C:UsersXymisDesktopтимпxml'sdata.xml", settings);
                doc.Save(writer);