If all you need is the prefix of the email you can achieve this result using a system module using the Users as the only dimension.
Create enough line items to parse the email to get the email prefix as outlined by @DavidSmith in this post. @kavinkumar has a good suggestion too but the formula should be broken up to keep it simple.
If you want a cleaner way to obtain the email address based on first name and last name I would recommend exporting the User list and re-importing into your own user list (just the email prefix - don't use list properties). Then I would create a system module using that list and import the First Name, Last Name, and their formal email address. Then once the user enters their email prefix, everything will auto-populate because you're line items will be formulated to look them up from the system module.