Do you mean that if email comes from root you want it to show a different from address? To do that you'd probably have to change each program which is sending email as root, or create some custom ACL for exim, to check all outgoing emails, and then based on certain criteria you define, rewrite one or more headers, depending on your needs. Neither is recommended.
Or do you mean email going to root? If so you create a .forward file in the /root dirctory.