General Message Board
I too have been wondering this. Found this info in the impossible to find rules tab on the fantasy baseball main page:
Got your eye on that hotshot prospect just called up from the minor leagues? This article explains when he will be eligible to pick up in your Yahoo! Sports Fantasy Baseball league, as well as why some AAA players are added into the system prior to their MLB debut.
Our Yahoo! Sports Fantasy Baseball experts always add a group of top prospects to the game before we launch for the season. Those players are available in every league from the beginning.
All remaining players are added to the game during the nightly system update following their addition to their MLB team's active roster.
While teams may announce their intention to call up a prospect, they may not make the actual transaction doing so until the day of their MLB debut.
The same is true of prospects who make an MLB team out of spring training -- they won't officially appear on an MLB roster until teams have to finalize their active rosters prior to their season opener. MLB teams can have 25 players on the active roster until Sept. 1, when all players on the 40-man roster are eligible to be used.
Because of this, if your league uses the setting "Wait until called up to the Majors," a prospect may not be available in the Fantasy game before their first actual MLB game is played, and they will be added to your league's available player pool subject to its waiver rules.
Note: We do not have the ability to add players in real-time.
To see a list of players who are added to the game, on your league homepage, go to the "Players" tab and click MLB Roster Changes to view the latest updates.
Players who show as "Activated" are available as free agents, as they already existed in the available player pool.