custom-converter.cs

using System;
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;

namespace jsonconvert
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
            var str = "{\"user\":\"{\\\"username\\\":\\\"test\\\"}\"}";
            var res = JsonConvert.DeserializeObject<Payload>(str);

            Console.ReadLine();    
        }
    }

    class User {

        [JsonProperty("username")]
        public string Username { get; set; }
    }

    class Payload {

        [JsonConverter(typeof(PropertyConverter<User>))]
        [JsonProperty("user")]
        public User User { get; set; }
    }

    class PropertyConverter<TProperty> : JsonConverter<TProperty>
    {
        public override TProperty ReadJson(JsonReader reader, Type objectType, TProperty existingValue, bool hasExistingValue, JsonSerializer serializer)
        {
            JToken token = JToken.Load(reader);
            if(token.Type == JTokenType.String){
                var u = token.ToString();
                return JsonConvert.DeserializeObject<TProperty>(u);
            }

            return default(TProperty);
        }

        public override void WriteJson(JsonWriter writer, TProperty value, JsonSerializer serializer)
        {
            serializer.Serialize(writer, value);
        }
    }
}