Changing the table may not be completely unreasonable, but does require caution: understanding database, making full backups, time offline during break and testing. If you have those abilities, then why not?
However, IP address list quiz access rule may be a better way to centrally administer the lists for you. It looks like it has the same restriction, but if you edit the file db/install.php before you install it to change the subnet LENGTH attribute to something larger, it will use that value when it creates its table. Actually you may not need to do that as you can break your list into several different named sublists and include multiple names on a quiz.