Is it possible to create a new player and add him into the draft class so I dont have to create a completely new draft class for one player?
or edit the ratings?

If you have the 2017 version I believe It works pretty much the same in the 2018 version. I don't yet have the 2018 version so I can't exactly tell you where to look. I believe that If you search player you create a new player that way and there should be a box where you decide If this is a player for the draft or just a free agent.