This is the line that sets the userif (I think) - line 652 in infraction.php:
PHP Code:
$infdata->set('whoadded', $vbulletin->userinfo['userid']);
So, look at the file and see if there is a hook after that, but before the infraction is actually given, and change that variable.
You should also look at the PM processing underneath that line and see about modifying who the PM is from also. These are the two lines for that (around line 747):
PHP Code:
$pmdm->set('fromuserid', $vbulletin->userinfo['userid']);
$pmdm->set('fromusername', $vbulletin->userinfo['username']);
There is a hook right under those lines for you to use to change them.