PKCS # 12 is a standard developed by RSA for PKI (Public Key Infrastructure). It consists of a tree of certificates and the keys that accompany them. Your PKCS # 12 contains one leaf certificate and its key, possibly along with some higher certificates (this is an inverted tree).
PGP, which is used by Seahorse, is another way of processing public / private keys, based on a trust network (google it). It is not directly compatible with the PKI (X) that you have now. You can extract the key and generate a PGP key from it, but you still have to restore trust so you can use another key pair ...
source share