Add new item in xml file from object

I have the code below that generates a xml from the object. Now what I need is to open the generated xml file and add a new record below the record already inserted.

    string Arq = Path.Combine(Caminho, NomeArquivo);

    StreamWriter Wr = null;

    XmlSerializer x = null;
    try
    {
        Wr = new StreamWriter(Arq);

        x = new XmlSerializer(Objeto.GetType());

        x.Serialize(Wr, Objeto);

        Wr.Close();
    }
    catch (Exception ex)
    {
      Utilexcessao.TrateExcessao(ex);
    }
    finally
    {
        if (!(Wr == null))
            Wr.Dispose();

        if (!(x == null))
            x = null;
    }