How can I import a PGP public key into Keychain Access.app?
I think you're trying to mix two different encryption schemes here: S/MIME and PGP. Mail.app supports S/MIME by default, so I guess that's what you're currently using. PGP support can be added by installing the Mail plugin that is part of the GPG Suite. After you also generate a PGP key for yourself, you can use both S/MIME and PGP encryption. You can receive and decode both, and for each email you're sending, you can choose which one to use.

GPG Suite contains a separate GPG Keychain app for PGP keys.