Change the email address in VF02 before saving

Hi Experts,

I have a requirement in which I have to change the email address when the user selects my custom output type in VF02 transaction .

The process :

VF02 - Enter a billing document number - Go To - OUTPUT - Select the desired OUTPUT TYPE which has transmission medium as External Send - then Save-

after this my requirement demands to give a pop up in which the user will enter his email id and then the mail goes to the email address entered by the user in the popup.

so I would like to know if there are any user - exit available for this scenario and how to proceed further.

Thank you in advance