CSV driver doesn't support data writing (as well as other DDT drivers), you need to open the file as usual text file and write raw data into it.
To build on what karkadil has indicated, there are many ways of doing precisely what he suggests. One such way is to use the aqFile scripting object. The way I'd do it is
1) Open the file to be read using aqFile.OpenTextFile
2) Open another new file as writable for your output
3) Use ReadLn to read the columns and immediately write it out to the new file
4) Start a "while not end of file" loop
5) Use ReadLn to read a line of data
6) Modify what is read in to include your value you want for the third column
7) Use WriteLn to output to the new file
8) After the loop is finished, close both files.
