Yes this is possible and documented in the mailcow.conf file. In order to add imap.domain.de and smtp.domain.de to your Mailcow, you can simply uncomment the already existing example line in the section “Additional SAN” of the file…
# Additional SAN for the certificate
# You can use wildcard records to create specific names for every domain you add to mailcow.
# Example: Add domains "example.com" and "example.net" to mailcow, change ADDITIONAL_SAN to a value like: