Okay, so I've found the one thing I
can't change.

Since I'm going to do this as an offline fantasy game, I can't guarantee I'll have 30 teams (in fact, I'm expecting 8 to 12).
What I really want is a fictional baseball league with major league rosters for the inaugural draft. That's it. I want full control over number of teams, league structure, everything else, just with the major league rosters. Is that possible?