Hello,
Thanks for reaching out. Impersonation is already always logged. Systems Administrators can find the logs regarding user impersonation in the Administrative Log. You can find this under Manage->Troubleshooting->View logs. Another thing to note. Only system administrators can impersonate. The most that a domain admin can do is view the password but that is logged as well.
Regards,
Zach Sylvester
System/Network Administrator
SmarterTools Inc.
(877) 357-6278
www.smartertools.com