XmlDocument document = new XmlDocument();
document.Load("contosoBooks.xml");
XPathNavigator navigator = document.CreateNavigator();

navigator.MoveToChild("bookstore", "http://www.contoso.com/books");
navigator.MoveToChild("book", "http://www.contoso.com/books");

XmlWriter pages = navigator.AppendChild();
pages.WriteElementString("pages", "100");
pages.Close();

Console.WriteLine(navigator.OuterXml);

XmlDocument document = new XmlDocument();
document.Load("contosoBooks.xml");
XPathNavigator navigator = document.CreateNavigator();

navigator.MoveToChild("bookstore", "http://www.contoso.com/books");
navigator.MoveToChild("book", "http://www.contoso.com/books");

navigator.AppendChild("<pages>100</pages>");

Console.WriteLine(navigator.OuterXml);

XmlDocument document = new XmlDocument();
document.Load("contosoBooks.xml");
XPathNavigator navigator = document.CreateNavigator();

navigator.MoveToChild("bookstore", "http://www.contoso.com/books");
navigator.MoveToChild("book", "http://www.contoso.com/books");

XmlReader pages = XmlReader.Create(new StringReader("<pages xmlns=\"http://www.contoso.com/books\">100</pages>"));

navigator.AppendChild(pages);

Console.WriteLine(navigator.OuterXml);

XmlDocument document = new XmlDocument();
document.Load("contosoBooks.xml");
XPathNavigator navigator = document.CreateNavigator();

navigator.MoveToChild("bookstore", "http://www.contoso.com/books");
navigator.MoveToChild("book", "http://www.contoso.com/books");

XmlDocument childNodes = new XmlDocument();
childNodes.Load(new StringReader("<pages xmlns=\"http://www.contoso.com/books\">100</pages>"));
XPathNavigator childNodesNavigator = childNodes.CreateNavigator();

if(childNodesNavigator.MoveToChild("pages", "http://www.contoso.com/books")) 
{
    navigator.AppendChild(childNodesNavigator);
}

Console.WriteLine(navigator.OuterXml);