简单粗暴，直接上代码：
XmlDocument xmlDoc = new XmlDocument();
xmlDoc.Load(Help.basePath);
XmlNode root = xmlDoc.SelectSingleNode("//cusBanNetList");
XmlElement timelock = xmlDoc.CreateElement("BanNetList");
timelock.InnerText = txtBanNetworkUrl.Text;
root.AppendChild(timelock);
xmlDoc.Save(Help.basePath);
Help.basePath------------文件路径
cusBanNetList----------节点名
txtBanNetworkUrl.Text---------------节点值