Sorry, I'm not quite sure I understand your requirements. Do you already have an SFTP/SSH server up and running, or are you setting it up? In that case, you would have to export the server's public key from the server software and share that with the clients. You must not share the server's private key with the client, because that would defeat the whole purpose of SSH. Or are you trying to have your clients authenticate to your SFTP/SSH server using private key instead of a password?