You want to Use the same DKIM key on multiple Messaging Gateways that support the same domain.
The Messaging Gateway does not have an export button for Domain Keys.
Messaging Gateway
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