How can I represent keepers on the draft board?

There's no way to place keepers on the draft board (yet) but commissioners can delete the draft pick that is assigned to the keeper. The owner will lose their pick in that round, and one will get added to the end of the draft so that they still get all of the picks they need to fill their roster.

To remove a pick that represents a keeper in the draft, commissioners should go to League » Settings » Edit Draft Order and choose the "Edit Rounds" button at the bottom. Select a round to edit, and then use the drop-down in the Owner column to select the [Delete Pick] option from the list and then Save. The owner will lose their pick in that round, and one will get added to the end of the draft so that they still get all of the picks they need to fill their roster.