Say I want to add a prospect that would be drafted in maybe the 3rd season. How would I do that?
I'm not sure if you can get one into the 3rd season, but what you want to do is create a player, set their birth year to 1997 (youngest you can), make sure that they have no pro team, and that they're undrafted (sometimes you can't scroll through these, so I just hit random until I get those 2 attributes set, then redo the age and name and stuff), set all their other attributes like height, weight, hand, position, and stats, then hit create.
Next you want to go to roster management, player movement, and find your newly created player in the list of free agents. Put them in a junior team, then start your Be A Gm with your new rosters.
There may be an easier way, but I have not come upon it yet