How to sync certificates generated by external Root CA with internal CA

Hello All,

I have 3 zones (internet, intranet and DMZ). Internet and intranet zones are physically separated and in different domains. I need to build a stand alone root ca in DMZ zone common for both internet and intranet zones and this root ca in dmz zone has to be able to sync/import certificates generated by external root ca. How to achieve this. 

Thanks