This coming Sunday, September 1, the Cubs (and the 29 other MLB teams) can expand their active rosters beyond the 25-man limit to as many as 40 players.
This will be the last season teams can carry that many players in September. MLB and the Players Association agreed last winter to have a 26-man roster (with a 13-pitcher limit) from Opening Day through August 31, then a 28-man roster limit after September 1. My hunch is that the 28-man limit is too small and it will be increased, perhaps to 30, this winter.
But for now, teams can load up on extra players without restriction up to their 40-man roster limit.