On the Free Agent page - on the far right - in the drop down menu ACTIONS - second line is Import Historical Players.
click on that link and enter the database you're using (most like default one is "historical_database.odb") click on that database and hit confirm.
you will need to know the players ID for the game at this point to enter it in the game. All the players ID are in a .csv file in your stats folder. (usually under C:\Documents>Out Of the Park) i have mine changed to D:\Documents\Out Of The Park Developments\OOTP Baseball 20\stats - under the .csv file Master) the B column has the player id's (bondsba01).... another place to find them is on Baseball Reference. It would be in the URL of the player -
https://www.baseball-reference.com/p...ondsba01.shtml - the bondsba01 is the part you'd need to copy into that first box. and the add the year of his import, ie bondsba01, 2001
then you
need to enter which year of his career you want the import to start at. (i just used 2001 as an example. 1986 was his rookie year i think. so you'd need to change 2001 to 1986 is all.)
i always check include players in the minor league database as well. then decide if you want selected stats included and if you want him to go into free agent pool or the draft pool.
if you want him on a specific team, you'd need to include the 3 letter team abbreviation of that his, preceded by a coma in the first line
bondsba01, 2001, SFG (i think that's the Giants id, but i'm not at my game computer, so i'm guessing on that one.)
if all goes well, and everything is entered correctly, he should appear either free agent pool, draft pool (which is only visible a few weeks before your draft date) or on a team if assigned.
good luck. hope this helps. as it's off a fading memory, i hope i didn't miss something or incorrectly remember.
if you need to import more than one player at a time - pm me and i can dig up a write up i have somewhere in a file on how to import from a text file and can send that to you.