Survey Results from PX not showing in CS

Hi Team,

While customer imported the survey from PX, In PX, email is not mandatory to send email but in CS, email is mandatory one without which survey cannot be imported. The system is designed to use  PX User Id for mapping ( PX -> CS ) to import surveys from PX -> CS.  Can you provide any alternative solution or can we make Email as mandatory in PX while imported.

Error: No email id is associated with px user with id