Follow the steps below to export your CPAC (which was already installed on Keychain into a PKCS12 file).

  1. Navigate to Applications > Utilities > Keychain Access
  2. In the Keychains options (on the left), select Login and click My certificates in the Category panel.

    keychain access

  3. Next, select the certificate you want to export ad click File then Export Items:

    export items

  4. Now, for the File Format, select Personal Information Exchange (.p12). Name it as you wish, and save it in a directory of your choice.

    file format

  5. Next, create a password for the exported file. It will be requested if/when you import the certificate into another browser/mail client or device.

    password

  6. Click OK. You have successfully exported your Sectigo Personal Authentication certificate.

Once you’ve exported the Email;/Personal Authentication certificate into P12 format, you can import it into a MAC OC using Keychain Access. To complete the process, follow the steps below:

  1.  Go to Applications > Utilities > Keychain Access
  2. In the Keychains panel on the left, select Login > File > Import Items…

    Import Items

  3.  Now, locate your saved certificate file and click Open.

    enter password
    Note: If prompted to trust certificates issued by your CA automatically, select the Always Trust option to trust and install your certificate.

  4.  You can view the installed certificate by clicking Category > My Certificates in the Keychain Access window.

Source: Sectigo’s Knowledge Base