[exim] Address Rewriting - getting the "$1"

This might have been discussed before, but searching the archives
for it wasn't easy, I swear.

I would like to rewrite the To: header, at SMTP time, for a few users.
The local_parts are in a cdb file and can be extracted by:

STAFF = ${lookup{$local_part}cdb{moronic_users.cdb}{yes}{no}}

The staff are currently using XYZ@???.

What I want is that for each staff user, I'd like to rewrite the
address to XYZ@???, as in

wash@???    $1@???    STh

How do I get the $1 in this case?

        cheers
       - wash 
+----------------------------------+-----------------------------------------+
Odhiambo Washington                     . WANANCHI ONLINE LTD (Nairobi, KE)  |
<wash at wananchi dot com>              . 1ere Etage, Loita Hse, Loita St.,  |
GSM: (+254) 722 743 223                 . # 10286, 00100 NAIROBI             |
GSM: (+254) 733 744 121                 . (+254) 020 313 985 - 9             |
+---------------------------------+------------------------------------------+
"Oh My God! They killed init! You Bastards!"  
                         --from a /. post