Dear POS - Retrospectively add customer to sale

Problem:

POS user forgets to add customer to sale and processes the sale as walk-in. It's not possible to go into sales history and add a customer to the sale. We want the ability to track customer's sales history so would like to add a customer to a sale if this was not initially added.

Proposed feature:

Retrospectively add a customer to a sale that has been closed as a walk-in