Adding your own columns to PokerTracker database tables is not supported or recommended, and should only be attempted if you know what you are doing. We will not be able to help you if you have problems in an altered database.
That said, I believe that some users have made this kind of change - you should make a
Backup of your database before you attempt any changes, though, and be prepared to re-import to a new database if you have any problems.