[imp] Forcing an identity/from address

[Andrew Morgan]

I have IMP v3.2.2 configured so that my users cannot set their From:
address (trouble-maker students!) as follows in imp/config/prefs.php:

$_prefs['from_addr'] = array(
    'value' => '',
    'locked' => true,
    'shared' => true,
    'type' => 'implicit'
);

With this, the From: address is built from the username and realm, and
this works great.

However, I'd like to specifically set an alternate From: address for a
small number of users.  We would like to migrate them from an old email
system onto ours, but it is important that their outgoing email address be
the same as before.

I tried temporarily remove the lock on from_addr, setting an email
address, then setting the from_addr lock again.  The From: address worked
when I removed the lock, but once I locked the preference again, it went
back to username at realm.  I guess the code overrides the saved From:
address when the preference is locked?

Is that the way it is supposed to work?  Is there some other way to handle
this inside IMP short of modifying the code?

Thanks,
	Andy