MultiChain uses the same ECDSA cryptography scheme as bitcoin and bitcoin-derived blockchains, so I don't think it's possible to convert GnuPG keys to MultiChain directly.
Note however that in many cases it's possible to securely associate a non-ECDSA public key with a blockchain addresses by publishing the public key from that address onto a stream. Other nodes can then use liststreampublisheritems to retrieve the non-ECDSA public key for that address from the stream.
As for HSM, any HSM module that is compatible with bitcoin should be easily modifiable to work with MultiChain, since both the cryptography and transaction formats are identical.