Clerber Saad, Brian is correct in that SmarterMail uses it's own internal password ciphering.  You would not be able to migrate user accounts with just the MD5 passwords.  One workaround is to keep your old server running on a separate IP and have your users from the new SM server manually migrate their accounts over IMAP.

 

EDIT: Also note that MD5 encryption is one direction; once you have the MD5 output you can't go backwards to retrieve the original password.