Here are some experiments with different secp256k1 node libraries.
ssb-server.jsdemonstrates signing on secp256k1 and ed25519 on the same instance of sbot
index.test.jsdemonstrates using node-secp256k1 to generate keys and create Diffie-Hellman shared secrets, and ethereumjs/keythereum to show the corresponding Ethereum addresses.
elliptic.jsdemonstrates using elliptic to generate keys and do Diffie-Hellman.
private-box.jsis an implementation of scuttlebutt's private-box which works with secp256k1 keys and almost with a mixture of recipients having both curve25519 and secp256k1 keypairs. Highly experimental and not recommended for use in the wild.