Not that MLB 13 The Show has no roster support, it's just limited in what it can do in the minor leagues. That's where Operation Sports' roster editors come in—every single year.
See, there is no group license for real minor leaguers, and they aren't covered by the MLB Players Association until they get their first day of service in the big leagues. So for those who wish to make a realistic tour of the minors in Road to the Show, or manage the actual farm system of their favorite club in Franchise, the Operation Sports Full Minors custom roster is the only way to fly.
These guys edit and create hundreds of minor leaguers, which is as time consuming as it sounds. The game released on March 5. This is actually the second update to the Full Minors roster, catching some stray players and free agents who have moved around recently.
The file can be downloaded here, or look for it in the Roster Vault in the game's main menu. The file name is O.S.F.M. v2 Opening Day.