using System;

using Newtonsoft.Json.Linq;

public class Program

{

    public static void Main()

    {

        string json = @"[

            {

                ""data"": {

                    ""_hash"": null,

                    ""kind"": ""ENY"",

                    ""id"": ""t123"",

                    ""payload"": {

                        ""r:attributes"": {

                            ""lok:934"": ""@0|I""

                        },

                        ""r:relations"": {

                            ""lok:1445"": ""15318"",

                            ""lok:8538"": ""08562""

                        },

                        ""r:searchData"": """",

                        ""r:type"": [

                            ""5085""

                        ]

                    },

                    ""type"": ""EQT"",

                    ""version"": ""d06""

                }

            }

        ]";

        var arr = JArray.Parse(json);

        var payloadNode = arr[0]["data"]["payload"];

        var objectToAdd = new JObject(

                              new JProperty("r:Porelation", 

                                new JObject(

                                    new JProperty("r:ClassA", new[] { "123", "456" }), 

                                    new JProperty("r:ClassB", new[] { "Stuff", "Thing" }))));

​

        payloadNode["r:searchData"] = objectToAdd;

        Console.WriteLine(arr[0]);

    }

}