Now I have a smartcard (PKCS#11 / CSP interface via OpenSC) on which I have a 2048-bit RSA private key. I have a copy of OpenSSL on my machine as well as XCA (which supports the usual formats SSH2 / PEM / DER) with private-key access.
My aim is to add the public key, corresponding to the private key as an encryption subkey to my existing software-based keyring with an 4096-bit primary software RSA key. It would also be sufficient to add it "normally" as "somebody else's public key".
Now my question is:
How can I convert a PEM / SSH2 / DER encoded public key so that it will be accepted by GPG as a public RSA key?
Please note: "The key formats serve different purposes" is not a valid answer
If you want to suggest "use gpgsm" as an answer, please document how to do so with duplicati.