Hello @JohnP1864 ,
To do something like that, you'd need to create a new list, create the automation with a setup for new list joins to that new list, then make the automation live. Once the auto is live, you can go into the previously existing list, select everyone, and add them to the new list.
Another potential way to do it would be to setup an automation series that triggers off of a specific regular email being opened or a specific link being clicked, then send the applicable regular email to your existing list. Then you can make the automation live, and it'll start triggering to send to contacts that meet the criteria.
Otherwise the only other method would be to just create several regular emails and send them to the list at regular intervals.