Based on your supplied code:
private void AppendItems()
{
string subject = "TestSubject";
List<string> listItems = new List<string>();
listItems.Add("Items5");
listItems.Add("Items6");
XmlDocument xDoc = new XmlDocument();
xDoc.Load("test.xml");
XmlNode subjectNode = xDoc.DocumentElement.SelectSingleNode("Subject['" + subject + "']");
foreach (string item in listItems)
{
XmlNode newItem = xDoc.CreateNode(XmlNodeType.Element, "Items", xDoc.NamespaceURI);
newItem.InnerText = item;
subjectNode.ParentNode.AppendChild(newItem);
}
xDoc.Save("test.xml");
}