Allow adding approvals on the fly?

I’m implementing Requisitions and have set up approval maps. But they want to allow the creator of a requisition to add an approver on the fly. These additions don’t have a pre-defined criteria, so adding to approval map won’t work. It’s a case by case scenario. I see there’s a + button on the Approvals tab, but it’s always disabled. Does anyone know if Acumatica allows this to be enabled without customization? I’ve looked into it, but haven’t found anything.