I do this frequently by seeding my start up fictional leagues with filled players as better players come in you see a more realistic league development IMO. As the league matures the athletes it attracts are better and better skilled.
To do this quicker I simply start a blank league, fill minors on all teams.... reset, release all, and redraft. All done and ready to go.
(If you set the Initial draft to 60 rounds you'll clean the entire draft pool and start with 0 free agents... adjust down to taste..)
If you couple this start up with a June 1 draft you'll find lots of the very best players in the draft making their ML debuts on June 2nd during the early seasons of the league.