[exim] rewrite subject conditionally on To: field

Greetings,

Mail to "joeuser@???", where target.edu runs exim, in some
cases was originally sent to an alias on another mail system which
forwarded them. Let's say that forwarding address was
"AKA@???". These appear on the exim machine with headers like:

Envelope-to: joeuser@???
From: sender@???
To: AKA@???
Subject: test

and are delivered to joeuser.

What exim configuration rules would one use to convert the subject
line on ONLY the messages originally sent to "AKA@???" from:

Subject: test

to

Subject: [AKA] test

?

All other messages should retain their original Subject line. The
purpose for doing this is to give a visual indication of the type of
message received - otherwise in most mail clients the AKA origin of the
message isn't exposed until the message is opened and more headers
are shown to the user.

These rewrites would apply only to this one user. Can this be done
in a .forward file or must it be entered in the general configuration
with some sort of conditional on the Envelope-to and To: headers?

Thank you,

David Mathog
mathog@???
Manager, Sequence Analysis Facility, Biology Division, Caltech