>>Is there a way to create a waitlist from a closed event?
However, you can create a second form for the waitlist. For example, while there is space for an event registration, you will provide the Event-registration form to people to fill out. Now, when you have reached the max number, you would close the form and your Closed Form message could include a message with the FormURL of the waitlist form. Now, the user can copy the waitlist-formURL to the browser and go from there.
Or you can embed the forms on your website, one or the other, whenever applicable.
Hope this helps,