With OOTP11 the they do not have to have stats in the batting.csv but there needs to be something in the batting2.csv (and so on for pitching and fielding).
This could NOT be done prior to 11. (I thank myself for getting this fixed for OOTP11
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(Warning : I have not fully tested this below scenario)
If you want players to debut according to whenever you want just make the debut date whatever year you want. As long as there is a line for that year in the batting2.csv you're fine.
But there is a catch, you're initial import will look in the 2.csvs for stats. If they're there it will create the guy and if he "debuts" after that year you will have a double.
Example = Don Mattingly
MLB Debut 1982, your debut 1980, G/S database 1979.
If you use ours as a base and you do an inital import in 197 he will be created as he has stats in the G/S database for 1979-1996. Now you progress to the 1980 rookies and he will import again. Two Don Mattinglys.
To avoid this you would have to remove all lines from the batting2,fielding2 and pitching2 csvs for all years for each player before the debut in your database.