I'm not sure if this exactly answers your question, but if you want to import players not already in the game, you can do that from the free agent screen by hitting the import historical player button at the bottom. Though this involves individual players only, not entire draft classes, and you have to find their individual ID in the Master.csv file, I think. This will work as long as the player has stats for the year you want to import them. I'm not sure what it will do, if anything, if you try to import a player during a year in which the guy had no stats, I've never tried that.
I hope this is what you are looking for, if not, hopefully someone else will know what to do, or even if it can be done.