First of all in linux the password is "salted" with the user name so simply switching hash's will not work. Thats pretty much password basics.
In linix there are 2 files which make up the status of a users password. /etc/shadow & /etc/password.
In anycase if you are already loged in with enough privs to change the password then I dont really see any reason to clear a user password. Either change the users password and then change it back when the audit is done or add yourself a new user.