WinForms C# add additional data to CSV line

I have created a couple of forms which all must work with a csv file. I can register different users and then login in a profile if it exists. I add the information with StreamWriter:

using (StreamWriter writer = new StreamWriter(filePath, true))
      {
           writer.WriteLine(username + "," + password);
      }

Then i read the information with:

using (StreamReader reader = new StreamReader(filePath))
            {
                while (!reader.EndOfStream)
                {
                    string line = reader.ReadLine();
                    string[] userData = line.Split(',');

                    usernames.Add(userData[0]);
                    passwords.Add(userData[1]);
                }
            }

And

 bool isFound = false;
            for (int i = 0; i < usernames.Count; i++)
            {
                if (usernames[i] == username && passwords[i] == password)
                {
                    isFound = true;
                }
            }

I want to add other information to the line like picture location,amount of money,amount of garage space,but everything i've tried from the internet writes the data on a new line,not to the existing line. I got a few tips from our lector-to first read the file and then write to it,but it still doesn't work. Whenever i add the data i use

    using(StreamWriter writer = new StreamWriter(filePath,true))
{
     writer.Write("the username from the register from" + "," + "the password from the register form" + openFileDialog.FileName("the location of the file/What my lector told me to use"));
}

I only need to know how to add that additional data so i can proceed with the rest of my project. Im sorry if this question has already been answered and thank you for your time.