Display (editable) field for payment_date in back-end (Subscription Manager:)

Hello,

I'm trying to keep the EB database correct with actual payments made into a bank account.
At the moment the 'payment_date' is the date/time that I activate the subscription.
It is OK to do this
BUT,
would it be possible to add an editable field under "Payment Information" which allows me to change the date/time after I have "Apply"ed the change to 'active'?

I only use the off-line payment method and can't look at the bank account every day, so I do it on Fridays.
I need the 'payment_date' to be the actual payment date.

Thanks,
-Graham-