To import Standalone offline Root CA certificate to Enterprise Subordinate CA's Trusted Root AA store, follow these steps.
Copy the Root CA certificate from the Standalone Root CA we have created before.
Open the "Certification Authority" MMC console from Administrative Tools. (Start > Programs > Administrative Tools > Certification Authority). Select Properties from Context menu.
Click View Certificate button.
The Root CA certificate will be open and you can see the certificate is issued to "OMNISECU ROOT CA" and is issued by "OMNISECU ROOT CA". Only Root CAs can self certify.
Click the "Details" tab and click "Copy to file" to export the Root CA certificate to a file. After exporting the Root CA certificate, bring it to the Enterprise subordinate CA. Open the certificates MMC console (Start > Run > mmc). Open menu path File > Add/Remove Snap-in.
In "Add/Remove Snap-in" dialog box, click Add.
Select "Certificates" snap-in, and click "Add".
Select "Computer account" radio button, and click "Next".
Select "Local computer", and click Finish to open Certificates MMC console Snap-in.
Right click Certificates folder inside "Trusted Root Certification Authorities", and select "All Tasks > Import" from the context menu.
This action will fire "Certificate Import Wizard". Click "Next" in Welcome screen.
Select the physical location of the Root CA certificate to import. Locate the file in Open dialog box and click "Open".
In "Certificate Store" screen, select the default selection and click "Next".
In "Completing the Certificate Import Wizard" screen, click "Finish" to finish the import.
"The import was successful" message will be displayed.
Open the "Certificates" folder inside "Trusted Root Certfication Authorities" Tree in "Certificates" MMC console. Check whether you can locate the newly imported certificate there.
You have imported the Root CA certificate inside "Trusted Root Certification Authorities" store.