Go to the individual player importer (Free Agent screen, far righthand side). Instead of the code it tells you to put in via the example, use the Historical Minors ID of the player you want. Here is a thread on how to find that code:
https://forums.ootpdevelopments.com/...d.php?t=340891
Use that code and then immediately under where you put the code and year, there is a checkbox to also search/utilize the Minor League db. Check that box. This brings in anyone in the Minors db that never made it to the bigs.