xml文件insert3.txt

c#的知识类库

如下：
原数据：
<?xml version="1.0" encoding="utf-8" ?> 

<Expressions>

	<Expression ID="1" name="a+b" remark="测试一">
		<Param name="a" remark="注解1"></Param>
		<Param name="b" remark="注解2"></Param>
	</Expression>
	
</Expressions>

如何操作使该文件内容变成：
<?xml version="1.0" encoding="utf-8" ?> 

<Expressions>

	<Expression ID="1" name="a+b" remark="测试一">
		<Param name="a" remark="注解1"></Param>
		<Param name="b" remark="注解2"></Param>
	</Expression>
         <Expression ID="2" name="x-y" remark="测试二">
		<Param name="x" remark="注解1"></Param>
		<Param name="y" remark="注解2"></Param>
	</Expression>

	
</Expressions>
希望能给个例子，谢谢！！
//------------------------------
string sFileName = "ExpXml.xml";
		XmlDocument xmldoc = new XmlDocument();
		xmldoc.Load(sFileName);
		XmlNode node = xmldoc.DocumentElement.ChildNodes[0].CloneNode(true);
		node.Attributes["ID"].Value = "2";
		node.Attributes["name"].Value = "x-y";
		node["Param"].Attributes["name"].Value = "x";
		node["Param"].InnerText="aaaa";
		//......
		xmldoc.DocumentElement.AppendChild(node);
		xmldoc.Save(sFileName);