Here is a thread where it is discussed (also see the manual, accessible from the link at the bottom of the start screen on OOTP).
https://forums.ootpdevelopments.com/...d.php?t=340424
Here is how to do it en masse (but you still need to make the file):
https://forums.ootpdevelopments.com/...d.php?t=318557
There is a toggle on the import to put them in the draft class as opposed to being a free agent (default).
It's the "historical_database.odb" you want. You do it after the game is created. From the Free Agent list screen, far upper right drop down.