I don't think it's possible to do what you are asking because of the way keys are generated for use in ECDH versus Ed25519.
Consider a Wireguard key pair (
pk1). Note that
sk1 is just 32 random bytes with the appropriate bits set/cleared (source) and that
pk1 is derived from
sk1 in the typical ECDH manner (source).
For Ed25519 signatures, you need to hash the 32 random bytes to produce 64 bytes, half of which are used to derive the public key the other half of which are used when creating signatures.
So you could derive a private key
sk1 and use
sk2 to create Ed25519 signatures. But based on the discussion in the comments, it sounds like you want to verify these signatures without access to
sk1 (presumably at the other end of the Wireguard tunnel, which only has
pk1). And since you can't derive
pk1, you can't calculate
pk1. Therefore you can't derive