Hi,
Where exactly you need this change ? In the invoice, or emails or both ?
What can be done is create a custom field in the table "order", don't display it in frontend, in order to not let the customer enter anything in that field. Then in the order management section in the backend, fill a value for this custom field.
And finally with PHP code, display the desired string with the desired value by getting it from the custom field.
The code can be different depending on the view we are. As it is a customization and require PHP knowledges, we can't really help you on this apart from giving you indications on how to achieve that