The only thing about the playoffs you can edit is where the game takes place.
By using the schedule editor, you can change which team has the home field for any game, or you can add extra off days (or remove them) to the playoff schedule.
So if you wanted to change the home field rotation for a best-of-7 series to something like 2-2-1-1-1 or 3-4, you could do that. But you cannot change the length of the series, that's controlled by OOTP.
Hopefully the next version of OOTP will allow us more flexibility in terms of setting each playoff round's length.