Customer have created the Domain Key by clicking on Add button. How to export a domain key which is added in SMG Administration > Settings > Certificated > Domain Keys. Customer need to import this key into another SMG to implement DKIM signing for outgoing emails.
Unable to import the key.
It doesn't support.
If customer want to share the domain keys, you need to create one through CLI and then convert to PEM format so it can be import to BCC.
A single domain can support multiple keys, by using different selectors.