Appending XElement to existing XDocument

I have the following XML string in database.

<?xml version="1.0" encoding="utf-16"?><ServiceList><Service><COMPAT>2</COMPAT><EQUIPID>0</EQUIPID><TITLE>Collect Call and SMS</TITLE><SMSCOMMAND>0</SMSCOMMAND><DIALCOMMAND>123</DIALCOMMAND><DEACTIVATIONCOMMAND>0</DEACTIVATIONCOMMAND><MODE>Dial</MODE><DETAIL>Here you go.</DETAIL><IMAGE>2014-16-9--16-28-25</IMAGE><LONGDESC><![CDATA[<p>P<br />P<br />P&nbsp;</p>]]></LONGDESC><Mechanism><Title>Mech Title</Title><Description>Here you go.</Description><Trigger>Mech Trigger</Trigger><Controls>1</Controls></Mechanism></Service></ServiceList>

i needs to add/appened following element inside ServiceList

<Service><COMPAT>2</COMPAT><EQUIPID>0</EQUIPID><TITLE>Collect Call and SMS</TITLE><SMSCOMMAND>0</SMSCOMMAND><DIALCOMMAND>123</DIALCOMMAND><DEACTIVATIONCOMMAND>0</DEACTIVATIONCOMMAND><MODE>Dial</MODE><DETAIL>Here you go.</DETAIL><IMAGE>2014-16-9--16-28-25</IMAGE><LONGDESC><![CDATA[<p>P<br />P<br />P&nbsp;</p>]]></LONGDESC><Mechanism><Title>Mech Title</Title><Description>Here you go.</Description><Trigger>Mech Trigger</Trigger><Controls>1</Controls></Mechanism></Service>

The c# code is given below where i am creating the document

            XElement ServiceList =
                new XElement("ServiceList",
                    new XElement("Service",
                        new XElement("COMPAT", "2"),
                        new XElement("EQUIPID", equipId),
                        new XElement("TITLE", form["Title"]),
                        new XElement("SMSCOMMAND", smscommand),
                        new XElement("DIALCOMMAND",dialcommand),
                        new XElement("DEACTIVATIONCOMMAND", smsdecactivationcommand),
                        new XElement("MODE", mode),
                        new XElement("DETAIL", form["Detail"]),
                        new XElement("IMAGE", Datetime),
                        new XElement("LONGDESC", new XCData(htmlstring)),
                        new XElement("Mechanism",
                                new XElement("Title", form["Mechanism.Title"]),
                                new XElement("Description", form["Mechanism.Description"]),
                                new XElement("Trigger", form["Mechanism.Triger"]),
                                new XElement("Controls", form["Mechanism.Controls"])
                            )
                        ));

            XDocument xml = new XDocument(ServiceList);

i simply convert the above document to string with the help of following method and insert it to database as string.

    public string GetXMLAsString(XDocument myxml)
    {

        StringWriter sw = new StringWriter();
        XmlTextWriter tx = new XmlTextWriter(sw);
        myxml.WriteTo(tx);

        string str = sw.ToString();// 
        return str;
    }