Is this possible? I have tried using a draft class from CF2020 but it was unbalanced (took their raw ratings from CF). Any advice is appreciated

Yea, export the draft class from CF2020, then import that into PF21. Usually I recommend using the "Adjust Draftees" checkbox which changes their ratings a bit to make them more like rookies but feel free to experiment with settings that work for you. After the import you can also use Mass Update if you wish to do wholesale changes to groups of rookies.

What stage of the game should I be importing the rookie class during? I want to try and bring some over frmo CFB20.

Thanks in advance!