For the most up-to-date answer to this question, you should refer to the HubSpot Academy Certifications FAQ for the most up-to-date answer to this question. Here is the answer as of 4/23/2018.
If you would like to change the email address or username associated with any of your HubSpot certifications, please submit this form with your information. Your request will be processed within one business day.
Because certifications cannot be transferred to a different email address, if the email address doesn't exist as a user in your HubSpot account, you will need to add this email address as a new user. You can sign up for a free Academy account using the new email address for the certification. Once your new account is created, you can have your certifications moved over by submitting this form.