We never generated P-521 keys, their use is not tested and not supported. We use SHA-256 for signatures, but P-521 keys require using at least SHA-512.
Importing the keys is not supported, see FAQ entry and the answer at
I also don’t understand why you want to use NIST P-521 keys, NIST curves are harder to implement securely than Ed25519 and NIST P-521 is just slower than NIST P-256.