You can upgrade from this add-on Unmaintained - Login as User. When updating a resource, all necessary and old add-on data is transferred to your forum.
This add-on gives privileged users the ability to login as any other user on the forum (as long as they're not whitelisted).
Key Features:
Permission controlled - choose who can login as another user
Whitelist - define users that cannot be logged into by others
Make users supply a reason when logging in as another user
Send a conversation when logging into another user
Doesn't cause your IP to be logged with the user being logged into
Alerts, conversation, threads are not marked as read.
All logins are logged to the moderator log