You don't need to get the passwords in plaintext to check them.
The passwords are not encrypted (encryped things can be decrypted ). They are hashed and salted.
You can get the hashed passwords with zmprov.
If you have them you take your weak password and hash it with the same algorithms (don't forget to use the same salt).
If the user hash and the weak password hash is the same you know that the user uses the weak password.
The salt is random so you have to recreate the weak password hash for every user with the users salt and check if the hashes are the same.
You can also make an dictionary attack with this, so you see which users should change their passwords.
Release 7.1.2_GA_3268.UBUNTU8_64 UBUNTU8_64 NETWORK edition.