Linking User fields to Timesheet Report

Is it possible to bring in fields in the User object such as email address in to a Timesheet report based on the reported by field? For example I want to add a coloumn to the Timesheet report called Employee Id that is brought in from the User Object. I know I can create a custom field with a reference to the user object. But how do I create a workflow that populates that custom field when time is entered?