I see that the users have a custom fields tab but I don't see a way to add fields? How do I do that?

I need to give users the option of buying 24hr access to a site. I was going to build roles for this and use a scheduled task to remove user access. But I think that might get clunky.

I would love to have a userType field instead of using a role for this. It would also be great to have an AccessEndDate field so I didn't even have to have much logic in my user cleanup task.

Or is there an even easier option for this already built in?