MTG CLM supports the integration of different Certificate Providers. For the direct integration of an existing external private PKI, the Microsoft CA Certificate Provider is currently the primary supported option. This allows an existing Microsoft PKI (AD CS) to remain in operation while being managed through MTG CLM.
A generic direct integration of arbitrary private CAs as Certificate Providers is currently not available. If you use a different private PKI, we would be happy to work with you to evaluate the available integration options for your specific use case.