using System;

using System.Linq;

using Newtonsoft.Json.Linq;

​

public static class Program

{

    public static void Main(string[] args)

    {

    var json = JObject.Parse(@"{""DanaerysTargaryen"":{""Dragons"":{""Dragon1"":{""Name"": ""Drogon""}},""Hair"": {""Color"": ""White""}}}");

    var toAdd = "DanaerysTargaryen.Dragons.Dragon1.Color";

    var valueToAdd = "Black";

    var pathParts = toAdd.Split('.');

    JToken node = json;

    for (int i = 0; i < pathParts.Length; i++)

    {

        var pathPart = pathParts[i];

        var partNode = node.SelectToken(pathPart);

        if (partNode == null && i < pathParts.Length - 1)

        {

            ((JObject)node).Add(pathPart, new JObject());

            partNode = node.SelectToken(pathPart);

        }

        else if (partNode == null && i == pathParts.Length - 1)

        {

            ((JObject)node).Add(pathPart, valueToAdd);

            partNode = node.SelectToken(pathPart);

        }

        node = partNode;

    }

    Console.WriteLine(json.ToString());

    }

}