As someone who has been testing the exact league you speak of (I try to not go to a draft till later in history), I came up with the idea of a Baseball Academy for young players. It isn't a feeder league because I didnt have much luck with feeders going into a league without a draft. It is just a fictional league with the league level set to "high school" and age minimum=13 and maximum=16.
So in theory when players turn 17 and their season is over they will become free agents for your baseball universe. After much work, I started to have success with this. After I built up a large pool of young free agents, I switched the Academy creation ages to just 13 or 14.
You will also need to set your Pro league to "allow free agents from other leagues" otherwise players will just get stuck in the Academy free agent pool with nowhere to go. I wanted my main league to not have financials for the early part of its history but you cant "allow free agents from other leagues" without financials on.
Hope this helps a bit and I encourage others to give their insight on how to make this idea work better!
