You may be able to use the PRTG API for this. You'll find all available API calls at "Setup > PRTG API". Are you looking to export the email address configured in a specific notification template or in the notification contacts of a specific user?
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.