You need "Dovecot Master User" - a user can read all users' mailboxes, and compose new email as that user.
Reference: http://wiki2.dovecot.org/Authentication/MasterUsers
NOTE: iRedMail supports Dovecot Master User by default, what you need to do is creating a user with command 'htpasswd' (search 'htpasswd' in above link), user accounts stored in file /etc/dovecot/dovecot-master-users-password. But you should understand how to use Dovecot Master User first.