1.) how do i add more domains (incl. postmaster etc)

You can use PostfixAdmin to manage mail domains.

You have to generate DKIM keys manually for every new mail domain. Search 'dkim signing' in FAQ page: http://www.iredmail.org/faq.html

Since you use MySQL backend, you can change 'enablesmtp' or 'enablepop3' or 'enableimap' to 0 if you need (0 means disabled).
It's NOT recommend to change the default mail related network ports.

i'm new into postfix/dovecot/iRedMail in general. so i still have some more questions and i couldn't find the answers yet (although i must admit for some questions i don't know which application i need to configure):

1.) how do i add more domains (incl. postmaster etc)
2.) are the dkim-keys then generated automatically or what do i need to do to generate the dkim-keys for the new domains
3.) how do i limit the access to smtps/imaps only? also is it possible to change the ports?

i know that i can change some iRedMail stuff via iRedAdmin but my backend is provided by MySQL, so it won't work.