using System;

using System.Xml.Linq;

using System.Xml;

public class Program

{

    public static void Main()

    {

                    string xml = @"<?xml version=""1.0"" encoding=""utf-8"" ?> 

  <UICustomizationSet xmlns:xsd=""http://www.w3.org/2001/XMLSchema"" xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"">

  <DefaultIcon>AvevaSharedIcons:ID_WARNING</DefaultIcon> 

  <UICustomizationFiles>

  <CustomizationFile Name=""Module"" Path=""design.uic"" /> 

  <CustomizationFile Name=""SchematicExplorerAddin"" Path=""CoreSchematicMenu.uic"" /> 

  <CustomizationFile Name=""Project"" Path=""$1.uic"" Optional=""true"" /> 

  <CustomizationFile Name=""SVGCompare"" Path=""SVGCompare.uic"" /> 

  <CustomizationFile Name=""Cabling"" Path=""AVEVA.design.cabling.uic"" /> 

  <CustomizationFile Name=""Hvac"" Path=""AVEVA.design.hvac.uic"" /> 

  <CustomizationFile Name=""Supports"" Path=""AVEVA.design.MDS.uic"" /> 

  <CustomizationFile Name=""Piping"" Path=""AVEVA.design.piping.uic"" /> 

  <CustomizationFile Name=""Steelwork"" Path=""AVEVA.design.steelwork.uic"" /> 

  <CustomizationFile Name=""MessageAddin"" Path=""MessageWindowCoreMenus.uic"" /> 

  <CustomizationFile Name=""Laser"" Path=""AVEVA.design.laser.uic"" /> 

  <CustomizationFile Name=""Integrator"" Path=""Integrator.uic"" /> 

  <CustomizationFile Name=""DiagramViewer"" Path=""DiagramViewer.uic"" /> 

  <CustomizationFile Name=""InstrumentationImportAddin"" Path=""InstrumentationImportAddin.uic"" /> 

  <CustomizationFile Name=""CAD Cable SYSTEM"" Path=""CADCableSystems.uic"" /> 

  <CustomizationFile Name=""CAD Piping"" Path=""CADPiping.uic"" /> 

  </UICustomizationFiles>

  </UICustomizationSet>

  ";

​

            var xd = XDocument.Parse(xml);

            xd.Root.Element("UICustomizationFiles")

                .Add(new XElement("CustomizationFile",

                        new XAttribute("Name", "Test Zone"),

                        new XAttribute("Path", "TestZone.uic")));

            Console.WriteLine(xd);

    }

}