How to Append a json file without disturbing the formatting

I'm working on a file to add/retrieve the data. The data format is JSON. I'm a newbie. I'm using JSON.NET to serialize and deserialize the data. this is the JSON format

{
  "EmpId": 1,
  "Name": "Kaushik",
  "Designation": ".net Developer",
  "JoiningDate": "09/23/2013",
  "Skill": [
    {
      "Id": 1,
      "SkillName": "C#"
    },
    {
      "Id": 2,
      "SkillName": "PHP"
    },
    {
      "Id": 3,
      "SkillName": "java"
    }
  ]
}

This is the JSON format I'm working on . Description Of the problem

1. I want to append the file data so I can add more JSON to this. For this, do I need to check if there is already some data or not?
2. I want to append the data in the JSON file. The appended form of data will look like this

[
    {
        "EmpId": 1,
        "Name": "Kaushik",
        "Designation": ".net Developer",
        "JoiningDate": "09/23/2013",
        "Skill": [
            {
                "Id": 1,
                "SkillName": "C#"
            },
            {
                "Id": 2,
                "SkillName": "PHP"
            },
            {
                "Id": 3,
                "SkillName": "java"
            }
        ]
    },
    {
        "EmpId": 1,
        "Name": "Kaushik",
        "Designation": ".net Developer",
        "JoiningDate": "09/23/2013",
        "Skill": [
            {
                "Id": 1,
                "SkillName": "C#"
            },
            {
                "Id": 2,
                "SkillName": "PHP"
            },
            {
                "Id": 3,
                "SkillName": "java"
            }
        ]
    } ]

My main problem is that I'm not getting the exact logic to append the file .

I'm reading the file char by char as follows:

int count = 0;
EmployeeDetail employee = new EmployeeDetail
{

    EmpId = ++count,
    Name = formCollection["Name"],
    Designation = formCollection["Designation"],
    JoiningDate = formCollection["JoiningDate"],
    Skill = new List<Skills>
                    {
                        new Skills(){Id = 1, SkillName = "C#"},
                        new Skills(){Id = 2, SkillName = "PHP"},
                        new Skills(){Id = 3, SkillName = "java"}
                    }

};

string json = JsonConvert.SerializeObject(employee, Formatting.Indented);
var dataFile = Server.MapPath("~/App_Data/json_data.json");
//Reading the file 
StreamReader reader = new StreamReader(dataFile);
int Tchar = 0;
char ch;
do
{
    ch = (char)reader.Read();

    Response.Write(ch);
    Tchar++;
} while (!reader.EndOfStream);
reader.Close();

StreamWriter file = new StreamWriter(dataFile, append: true);

file.WriteLine(json);

file.Close()