Yes, the existing Microsoft PKI can continue to operate and be connected to MTG CLM through the Microsoft CA Certificate Provider. The Microsoft CA remains the issuing PKI, while certificate processes can be centrally managed through MTG CLM.
Through MTG CLM, certificates can be requested from, issued by, and revoked through the Microsoft CA. This allows you to benefit from the centralized management and automation capabilities of MTG CLM without having to replace your existing Microsoft PKI.