知道怎么用winform创建一个XML文件了，就是不知道怎么继续向已有的XML文件里继续插入数据啊？

XmlTextWriter writer = new XmlTextWriter("hello.xml", System.Text.Encoding.UTF8);
                writer.WriteStartDocument(true);
                writer.Formatting = Formatting.Indented;
                writer.Indentation = 2;
                writer.WriteStartElement("Table");
                createNode(IDTextBox.Text, nameTextBox.Text, gender.Text, writer);
                writer.WriteEndElement();
                writer.WriteEndDocument();
                writer.Close();
                MessageBox.Show("XML File created ! ");

public static void addXmlTtem(string parentNode, string newNode, string filepath)
        {
            XmlDocument doc = new XmlDocument();
            doc.Load(filepath);
            XmlNode list = doc.GetElementsByTagName(parentNode)[0];
            XmlElement el = doc.CreateElement(newNode);
            //添加属性
            el.SetAttribute("属性1", "值1");
            el.SetAttribute("属性2", "值2");
            el.SetAttribute("属性3", "值3");
            ...
            ...
            ...
            //添加节点
            list.AppendChild(el);
            doc.Save(filepath);
        }

多谢！