If Alice encrypts a message with her own p, nobody other than Alice could decrypt the message.
But you could ask Alice to encrypt a message using Bob's public key so that Bob could decrypt it using his private key.
You should first understand how these things work and then try to use such tools, though to understand how they work you need to use the standard tools which already exist, GPG, PGP, are 2 useful tools.
Encrypting messages with bitcoin keys is possible but not a good practice. Let secp256k1 be used to store coins.
I think encrypting message with private key is fundamental operation of signing files.
Anyways as I searched, the Kleopatra (https://www.openpgp.org/software/kleopatra/) is good GUI tool for asymmetric encryption, also supports ECDSA NIST P-256 (not sure if it is secp256). but still it do not support encrypting with private key.