Hi
Per Keychain Access help menu:
To add a certificate to a keychain:
Drag the certificate file onto the Keychain Access icon or double-click the certificate file.
If you want to view the contents of the certificate before you add it, click View Certificates in the dialog, and then click OK when you are done.
Choose a keychain from the pop-up menu and click OK.
If you’re asked to provide a name and password, type the name and password for an administrator user on this computer.
For Keychain Access to recognize a certificate file, it must have a file extension that identifies it as containing certificates.
The following types of certificates are recognized by Keychain:
■PKCS12 DER encoded - extension .p12 or .pfx
■PKCS7 DER or PEM encoded - extension .p7r .p7b .p7m .p7c, or .p7s
If Keychain Access is open, you can add a certificate by dragging the certificate onto the Keychain Access icon in the Dock.
You can also add a certificate to a keychain by choosing File > Import.