SecItemImport
. You may find the SecPKCS12Import source code useful in understanding how to call it (though it's not that difficult if you understand other Security framework calls).
You may also be interested in this question, just to see some more code around this: Importing a PKCS12 Using SecItemImport
The PEM import should be slightly more straightforward, however, than the PKCS#12 version. You shouldn't need to even tell it whether it's ASCII armored; it'll typically figure everything out for you.