Convert single session to multi-session event

I highly suggest getting Zoom Event Professional Services involved. Maybe they can “do things” on the backend that is not possible for a licensed user to do. Be prepared to pay $300/hour, 3 hour minimum. 
https://explore.zoom.us/en/event-services/ 

If that’s out of reach for you:

Unfortunately, once the basic type of event has been selected in the initial step of the Zoom Events editor, it’s not possible to change it.

There are ways to do this, but require creation and publishing of a Multi-Session event, exporting your existing Single-Session registrations, communicating with your existing registrants that their registrations will be converted (you’ll want help constructing this email!), canceling the Single-Session event, then importing the registrations into your Multi-Session event.

Note that if this is a paid event (money collected by Zoom), the canceling will refund all money paid.

If you’d like my assistance, please reach out to me at the Z-SPAN link below.