class ItemConverter : JsonConverter
    {
        public override bool CanConvert(Type objectType)
        {
            return (objectType == typeof(Item));
        }

        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            throw new NotImplementedException();
        }

        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
        {
           new JArray(JObject.FromObject(value)).WriteTo(writer);
        }
    }
....

 string json = JsonConvert.SerializeObject(v, Formatting.Indented,  new JsonSerializerSettings { Converters = { new ItemConverter() } });

StringBuilder sb = new StringBuilder();
            StringWriter sw = new StringWriter(sb);
            using(JsonWriter writer = new JsonTextWriter(sw))
            {
                
                JObject root = new JObject();
                root.Add("message", new JValue("msg"));
                var p = new JObject();

                var oa = new JObject(); 
                oa.Add("lon", 12.0);
                oa.Add("lat", 14.0);
                p.Add("A",new JArray(oa));

                p.Add("B", new JArray(JObject.FromObject(new { lat=13.0,lon=16.0})));

                root.Add("p", p);
                root.WriteTo(writer);
            }

            string json = sb.ToString();