How often do you need to do this?
If you only need to do this once, then don't bother modifying the Moodle code. Just download the results file, and edit it in a spreadsheet program.
If you need this functionality a lot - well - it feels to me like there must be a better way to try to achieve whatever goal it is you are trying to achieve. What is the purpose of wanting to run your quizzes this way?