Attrib an email address to user

Hello,
I'm trying to configure the crm to allow a user that has been declared responsible for a task (support, etc.) to receive a notification. We have this option in crm/configuration/case/rules to send an email to the responsible. This one can be choosed in the users'list but there's no way to fix an email address to him ... I tried with several methods (an employee has an email address and can be linked to a user but the email is not following the link). so it will be good if we could add a field to the user's form to introduce his email.

Thnaks