using System;

public class Program

{

    public class MiClase

    {

        public string MessageId { get; set; }

        public string SerialNumber { get; set; }

        public object User { get; set; }

    }

    public static void Main()

    {

        var json = "{\"Id\": 1, \"Nombre\": \"Leches\"}";

        var obj = (object)Newtonsoft.Json.JsonConvert.DeserializeObject<dynamic>(json);

        var miclase = new MiClase { SerialNumber= "ABC", MessageId = "blabla", User = obj };

​

        Console.WriteLine(Newtonsoft.Json.JsonConvert.SerializeObject(miclase));

    }

}