Yes, (optional) playoff rosters are part of OOTP 12. We tried to mirror MLB rules as closely as possible.
Players on the DL or active roster on the day before roster expansion (i.e. 8/31) are eligible for your playoff roster. If a player gets put on the 60-day DL after that date, or on the 15-day DL during the playoffs, you may replace him with a player from within your organization prior to a playoff round, but only if that player was part of your organization on 8/31.
The playoff roster is a new section in the team transaction screen, you can move players via drag & drop as usual.
Things work pretty well
