Deploy an Internal CA Certificate

Been reading a lot of the previous questions and the option to deploy sounds easy by creating a GPO and going in to the following:

Computers Configuration -> Policies -> Windows Settings -> Security Settings -> Public Key Policies -> Trusted Root Certification Authorities and Right Click -> Import -> Point this to your root CA certificate.

My question is will this Root CA that I import and deploy via GPO be appended to those currently on the PCs provided in the Root Certificate Update or will it overlay those and start fresh?  Not certain if I need to extract a copy of what currently exists to populate the GPO and then add on the one Internal CA Certificate or if I only need the one I want to append?  Also going forward would the Root Update work hand and hand so that the PCs I want to deploy to keep up to date on those as well as the Internally Created CA Certificate.

  Thanks,
     Evan Cardanha