I thought they could be if they were in the organization already? AKA can't use a waiver claim, but could use Romy.. I probably am wrong though lol
Found this:
As mentioned, players acquired via trade in September are not eligible for their new team, and the same goes for those signed as free agents during that time. However, if a club wants to add a player who was not on the 40-man roster but was in the organization, it can petition the Commissioner's Office if that player is replacing someone who has been on the 60-day DL for at least 60 days.
One notable example of this was pitcher Brandon Finnegan, who appeared in every stage of the 2014 postseason for the Royals despite the fact that he was not added to the 40-man roster until Kansas City purchased his contract from its Double-A club on Sept. 1 of that year.