See here: That's not a database, dude!
CSV Files are just text: they have no "built in" sense of lines, much less columns - all of that formating information is added by the software that reads the data. All it has is a couple of values that are interpreted by software as control characters which say "this is the end of the line".
So you can't add a column directly because there is nowhere to put it! Instead, you need to read the CSV data into an application (either line-by-line or in its entirety), add the column to that data, and write the new rows back to a new file. You can then delete the original and rename the new file.
Even to insert or remove a single character in a file requires the same process: read the file data up to the insertion or deletion point while writing it to a new file, add or skip the character, then copy the rest of the file to the new file.
If you are going to start adding, modifying, or removing rows columns from your file you should really be using a database which is designed to do that.