In a regular trial, both sides (prosecution and defense) generally have an obligation to inform the other of any evidence (including witnesses) they plan to present at a trial, and to allow the other side to examine the witnesses or evidence before the trial starts. That process is called "discovery." It's not unusual for one side or the other to spring a "surprise" or "last minute" witness at a trial, but they have to justify the lack of notification to the judge. If they can't justify it, the judge can either refuse to let the witness testify, or declare a mistrial.

No such rules apply to a Senate hearing, because it isn't a trial. Besides, the Senate committee did notify Tom in (somewhat) advance, because, as we saw, Tom's "you've opened yourself up to five counts of perjury" speech occurred several days before Frankie took the stand.