Changing Drupal database
Bottom line: it's your database, you can add as many fields to whatever tables you like.

Drupal won't know anything about them, obviously, and it may cause unforeseen issues further down the line, as you're potentially changing the way the core system works. It's not possible to be any more specific than that without knowing pretty much everything about your application, so you'd need to test until you're happy you haven't broken anything.

The correct way to extend core entities is through the use of fields, I'd strongly recommend using those rather than hacking at database tables.