You can create your new/own DKIM keys with amavisd command (or "amavisd-new" on different Linux/BSD distributions). Details: http://www.ijs.si/software/amavisd/amav … .html#dkim
NOTE: iRedMail creates a DKIM key under /var/lib/dkim/ (or /var/spool/amavisd/dkim) during installation, it's enabled by default. And all virtual domains are signed with this key.