Is there a way as commissioner that I can set someone else’s keepers for them?
Yes, but only do this when all teams have set their keepers or else you will set keepers for all teams and delete all rosters. So do this right before you draft when the other teams have their keepers set.
League>>Settings>>Edit Rosters>>These are the keepers only (for all teams).
Then return to Edit Rosters when you're ready to draft and choose the draft mode (last and bottom option).