Customers that are created based on a sale or an event are "generated customers". A generated customer can be promoted to a full customer.
To promote a customer, open the customer 360 view, click the edit form and tick the box.
When a customer is promoted the "is_generated" flag is cleared from customer and if a customer numbering scheme (a list of single use tokens) is defined, a number is picked for the customer.
This feature can be switched on for you! Please contact email@example.com if you are interested.