If you have write permission to the file, then you should be able to add a column, but I would not be surprised to hear that an ALTER TABLE query would not work against a text file. You can use SQL to read those files, as you appear to know, but I'm not sure that ALTER TABLE is supported, or if it even makes sense for text files.

On the other hand, as long as you have write permission, you can write out the text file, replacing the one that is there. If you are working with lots of datatables or datasets, then it might make more sense to make the files XML files, as you would then be able to read or write the files into and out of datasets or datatables with considerable ease (it takes about one line for each operation). If you are stuck with the format, then go ahead and add the column to the datatable, as you are doing, but you will need to write out the file in a different fashion. I don't have any example of writing datatables to a delimitted file, that I can think of, but there are probably examples on this forum, so you could try searching for one. It would overwrite the file with the new information, though, so it would be a matter of reading in the entire file, making the changes, then writing the whole thing back out, including the changes.

If that won't work, somebody might have a suggestion for a workable ALTER TABLE query for text files. I don't KNOW that it is impossible, it would just surprise me a bit if it was possible.