Sleeper supports this case.
Make sure you set the league to Dynasty in league settings first.
The draft that you see when you create a league will be your first draft. You can modify the settings to "Vets" or "Rookies" only.
After this draft is complete, you can add a Supplemental Draft, which will put your league in pre-draft mode again, but preserving the teams / rosters from your initial draft.
This new draft can be your rookie draft, just modify the draft settings to Rookies only.
As long as you are in pre-draft mode, nobody can pick up rookies from the FA.
You can also do as many supplemental drafts as you want to keep adding to rosters, there is not limit.