I tend to agree with Bill on this and think that adding more fields to the SFSUsers table isn't likely a great solution. I'd tend to use another table (and actually do) for extra info.
However, I also agree with Edhy (

) that the UI for this could get messy. If you're using the SF security dialogs, then you'd need to provide another way to add extra user information. Or you have to replace the security dialogs to include the special user info. The first isn't ideal from the user's perspective and the second isn't much fun for the developer.
So, maybe the enhancement request would be extend the user form to allow an option extended user information. I can think of a couple of possible ways of doing this, both of which would just provide hooks into the form, such that additional information could be provided or accessed as needed. A couple of thoughts:
1. Have an optional toolstrip button on the user form which would call a delegate which would handle opening another form to capture/view additional information. The delegate would likely need the user_pk.
2. Have an optional extended panel avaialble, kind of like the browse dialog does. The panel could then have any UI needed and it would be integrated into the SF user form seamlessly. There'd have to be hooks between the panel and the main form, to manage saving of data, obviously.
Any thoughts of this?