how do i append to xml files

[abehm]

Hi,
I have a method that inserts class data into a new xml document, however I would like to append the data after the last element of an existing xml document.

here is the code I have:
if anyone can offer any help or suggestions, that would be appreciated.

public string Insert(string XMLFile)
    {
        string XMLDirectory = this.xmlDir + this.ProjectorSN + @"\";
        string file = XMLDirectory + XMLFile;
        System.Xml.XmlTextWriter writer;
 
        if (!System.IO.Directory.Exists(XMLDirectory))
        {
            System.IO.Directory.CreateDirectory(XMLDirectory);
        }
 
        writer = new System.Xml.XmlTextWriter(file, System.Text.Encoding.UTF8);
        writer.Formatting = System.Xml.Formatting.Indented;
        writer.WriteStartDocument();
        writer.WriteStartElement("ProductTest");
 
        writer.WriteElementString("ProductSN", this.ProjectorSN);
        writer.WriteElementString("PackageID", this.LightEngineSN);
        writer.WriteElementString("Part2", this.Lamp1);
        writer.WriteElementString("Part3", this.Lamp2);
        writer.WriteElementString("DateTime", this.DateTime);
 
        writer.WriteWhitespace(" ");
        // Write End of root element
        writer.WriteEndElement();
 
        // Write End of document
        writer.WriteEndDocument();
 
        writer.Close();
 
        return (XMLDirectory + XMLFile + " saved.");
    }