How can I enable add/drops before the draft?

Go to League » Settings » Edit Rosters and select These are the complete rosters.

Then when you're ready to draft, go to League » Settings » Edit Rosters and select These are the complete rosters but still draft this season. This will bring back the draft settings.

Note If you open the player pool early, the system thinks you have already drafted and you won’t be able to see the current season’s draft picks. However, they will be retained and you will see them again when you switch back into draft mode.