Asymmetric Encryption Using Existing Secret Key

I'm using Python to build a system in which users (accounts on a blockchain: defined by a public key and a secret key) need to communicate with each other securely and I think asymmetric encryption is the way to do it.

For example, if User A wants to send a message to User B, they encrypt the message with User B's public key and sends it to them. User B then uses their private key to decrypt the message and can read it.

However, I don't think it's possible (or I don't know how) to do this sort of encryption with the existing keypair I have for each account. Instead, I think what I should do is use a dedicated asymmetric encryption algorithm and generate the keypairs for that using the original keypair's private key as a seed/source (essentially, deterministically deriving a keypair with which I can do asymmetric encryption from the original private key).

Is this possible? Am I overcomplicating it? What libraries or algorithms should I look into (preferably Python-specific)? Thanks, any help is appreciated!

You almost certainly can use the the current key-pairs for encryption (e.g., via ECIES or RSA-OAEP). For hybrid cryptography, you're essentially encrypting the message with a symmetric cipher and encrypting that key asymmetrically.

Of course, you can also just generate new key pairs. You can deterministically generate new private keys from the original seed using a key derivation function, or you can just roll completely random keys.