The solution is simple. Identify the players and simply put a batting or pitching record into the database with all fields 0'd out. This will import them into your league initially. The hard part is identifying the players but you could look at the rosters of the year you want to import and go back 3 seasons, 5 if you are playing during the war years and go to baseball-reference and get a listing of all rookies for those years, then simply see if they are on your rosters for the year you are setting up initially. This would only take about an hour or so.
To those creating databases, this may be an option to put in, in future releases.