using System;

using Newtonsoft.Json;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        string json = "[{\"Id\":0,\"Nome\":\"\",\"Endereco\":\"\"},{\"Id\":1,\"Nome\":\"\",\"Endereco\":\"\"}]";

        List<JsonResult> items = JsonConvert.DeserializeObject<List<JsonResult>>(json);

        var last = items[items.Count - 1].Id + 1;

        items.Add(new JsonResult()

        {

            Id = last,

            Nome = "Leonardo",

            Endereco = "Rua de baixo"

        });

        string myJsonOutput = JsonConvert.SerializeObject(items);

        Console.WriteLine(myJsonOutput);

    }

    public class JsonResult

    {

        public int Id { get; set; }

        public string Nome { get; set; }

        public string Endereco { get; set; }

    }

}