Instead of a calculation field, make it a text field and auto enter the same calculation. Then you can edit the field.
So in effect it works as a copy and paste
With one huge difference: it does not use the clipboard so it does not destroy anything that the user has put on clipboard themselves.
As an aside: if you have the need to modify the result of the first field then perhaps look at alternatives to have that first field not be a calculated field. As Steve indicated, perhaps it can be an auto-enter calc, or perhaps it can be set in a scripted workflow instead.
Excellent advice. Being new with FM I wasn't aware of the differences between the auto-enter calculation and the regular calculation field entry but this advice has shown me a big difference, so thank you.