By default, draft classes are filled with randomly-generated players. If you're using a custom League File, that can include up to 3 draft classes of pre-defined players.
Additionally, it is possible to overwrite a draft class in any current league with pre-defined players. To do this, go to Players > Draft > Draft Scouting, click "Customize", and upload a Draft Class File for whichever year you want. When you do this, it will delete all of the current prospects for that draft class, insert players from the uploaded file, and (if necessary) randomly generate some new players to fill out the draft class if the file contains less than 70 players.
Like with the custom League Files, there are two places to get Draft Class Files: download one or make your own. Again like League Files, you might find some to download on Reddit. You can even use a League File as a Draft Class File - it will just take whoever is defined in the first draft and ignore everything else in the League File.
If you want to make your own Draft Class File, it is almost identical to defining custom players in a League File. The format is exactly the same. Just make sure to give every player a "tid" of -2. You can define as few or as many players as you want; if you define less than 70, then randomly generated players will fill out the draft class when people use it.