SqlConnection Conn = new SqlConnection();
SqlCommand Cmd = new SqlCommand();
XmlDocument OggettiInVetrina = new XmlDocument();
string Query = "SELECT * FROM Tabella FOR XML PATH, ROOT('Vetrina')";
Conn.ConnectionString = "Connessione";
Conn.Open();
Cmd.CommandType = CommandType.Text;
Cmd.Connection = Conn;
Cmd.CommandText = Query;
XmlReader Read = Cmd.ExecuteXmlReader();
Conn.Close();
OggettiInVetrina.Load(Read);
Random Rnd = new Random();
XmlElement RootVetrina = OggettiInVetrina.CreateElement("Rootvetrina");
int i;
ArrayList Row = new ArrayList();
for (i = 0; i < OggettiInVetrina.DocumentElement.ChildNodes.Count; i++)
{
Row.Add(OggettiInVetrina.DocumentElement.ChildNodes[i]);
}
for (i = 0; i < 9; i++)
{
RootVetrina.AppendChild((XmlElement)Row[Rnd.Next(Row.Count - 1)]);
}
XmlDocument Vetrina = new XmlDocument();
Vetrina.AppendChild(RootVetrina);
Vetrina.Save(Server.MapPath("prova.xml"));

The node to be inserted is from a different document context.

RootVetrina.AppendChild(OggettiInVetrina.DocumentElement.ChildNodes[Rnd.Next(OggettiInVetrina.DocumentElement.ChildNodes.Count - 1)]);

"SELECT TOP 1 [Descrizione_Breve],[Listino_1],[Listino_2],[Listino_3],[Sconto_Listino_1],[Sconto_Listino_2],[Sconto_Listino_3],[Titolo],[Immagine] FROM db_datawriter.Articoli WHERE [Codice] >= (rand() * (SELECT MAX(Codice) FROM db_datawriter.Articoli)) AND [Visibile] = 'Si' AND [In_vetrina] = 'Si' FOR XML PATH, ROOT('Vetrina')"