Generate secp256k1 ECC key pair with libsodium
Your question has been asked on the GitHub, too:

Is there some way with sodium/libsodium to make all crypto_sign_* methods use SECp256k1?

And the answer was given as

No, there isn't, and there never will be.
Libsodium is an opinionated API. You get one good algorithm for every use case. Fewer knobs and levers + more secure constructions = less crypto vulns in software that consumes the API.
For public key cryptography, you get ECDH and EdDSA over Curve25519. No Weierstrass or Koblitz curves.