Hi @CGESMike - Thanks for reaching out to us here on the Square Seller Community👋
I just tested this out on the app on my phone and didn't see an option to add a Customer to a past sale via the Transactions screen either. But if you visit your Square Dashboard online and go to the Transactions page, as mentioned in this thread, you'll see an option to add a Customer to the sale after you've clicked on the individual Transaction (shown below).
On the app, I did notice that you can add a Customer to the sale right after you've completed the payment. Otherwise, it looks like you'll need to do this from the Dashboard. P.S. I edited this thread to show my response as the Best Answer since this is the most up-to-date info.
I hope this information is helpful but please do let me know if you have any additional questions.
