using System;

using System.Linq;

using Newtonsoft.Json.Linq;

​

public class Program

{

    public static void Main()

    {

        string json = @"{

              ""Main"": [

                {

                  ""id"": 123,

                  ""Sub1"": [

                    {

                      ""id"": 2345,

                      ""description"": ""..."",

                      ""Sub11"": [

                        {

                          ""subid"": 6633,

                          ""xid"": 5555

                        }

                      ]

                    }

                  ]

                },

                {

                  ""id"": 332,

                  ""Sub1"": [

                    {

                      ""xid"": 5555,

                      ""description"": ""..."",

                      ""Sub11"": [

                        {

                          ""subid"": 6794

                        },

                        {

                          ""subid"": 4444

                        }

                      ]

                    }

                  ]

                }

              ]

            }";

        var root = JObject.Parse(json);

        var matches = root.Descendants()

                          .OfType<JObject>()

                          .Where(x => x["xid"] != null && x["xid"].ToString() == "5555");

        foreach (JObject jo in matches)

        {

            jo.Add("NewNode", new JObject(new JProperty("SomeProperty", "value")));

        }

        Console.WriteLine(root.ToString());

    }

}