So, you want to break out your drafts into two drafts (vets) first and rookies later, here's a step by step guide to accomplish that.
The option to create a supplemental draft will pop up once you finish your first draft.
First, ensure that your initial draft is completed. Then head over to the Settings tab and right beside "view results" you should see the option "Add supplemental draft".
Once you've add a supplemental draft, click the general setting icon > draft setting > scroll down until you see "Available players to draft".
Note: The default for supplemental drafts is all players.
On mobil, the "add supplemental draft" option is located within the league setting.
Once enabled, head over to the draft setting and scroll down until you see "available players to draft".