What’s the right way to amend a disciplinary write-up?

Q. "Recently we disciplined an employee for using the Internet without permission. A written warning was issued. The next day, the supervisor informed the employee that he would be on a two-week probationary period, which was verbally communicated to the employee. Since nothing was said or written about that probation period at the time of the discipline, is it possible to amend the write-up to include the probation period?" -- J.Q., North Carolina