Multiple Messaging Gateway Control Centers or Control Center + Scanner systems deliver mail for the same email domain and need to be configured with DKIM keys for that domain.
The Messaging Gateway does not have an export button for Domain Keys.
The Control Center operation to create a DKIM key does not allow export for use on other control centers. However, you can create the keys externally using utilities including but not limited to Windows key utilities or OpenSSL. The keys need to be in x509 PEM encoded format. Once the keys have been created, they can be imported into one or more Control Centers for DKIM use.
More information on the import is available in the following documents:
Certificate Settings - Domain Keys
Each SMG Control Center or Control Center + Scanner can be configured with its own DKIM keys provided DKIM is configured with a different selector with it's own DNS record. This is the standard means of handling multiple email servers with different DKIM keys.