how to add textbox data to csv file C#

hey i have a csv file that has data like this ( 1,1,asd,asd) and i want to add textbox data to the file so it ends up like this (textbox,1,1,asd,asd)
is this posiible and there is more that one row and needs to add it for every row ??
apperate the help !!

at the moment i  upload the file read its contents and insert it in a database using this code 
if (FileUpload1.HasFile) { string filename = Server.MapPath("Uploads/") + FileUpload1.FileName.ToString(); //getting complete path for saving the uploaded excel/csv FileUpload1.SaveAs(filename); // saving the csv bool saved = true; //creating a table to temp store the csv data DataTable tblReadCSV = new DataTable(); tblReadCSV.Columns.Add("CarID"); tblReadCSV.Columns.Add("HomeID"); tblReadCSV.Co ...