[Dovecot] Mapping username only for auth

I am running Dovecot alpha 3 with authentication from the local shadow file. I'd like to change the authentication to use our radius server.
I can probably do this through pam.

The usernames on the mail server are set up as first initial + last name ( jdoe for John Doe ). The usernames on the radius server are the employee number. Is there a way to map the username to the employee number in dovecot before handing off the authentication to the radius server? I want the usernames on the mail server to remain first initial

• last name. I don't mind maintaining a mapping file to go from username to employee number because I can automate this task.

Has anybody done this?