Unfortunately my understanding is that this is essentially impossible. SMB hashes the password on the client-side, and the hashing algorithm isn’t compatible with the algorithms used in /etc/shadow (it’s unsalted and less secure). I doubt Linux distros would want to have another field in /etc/shadow just for Samba passwords, and deal with keeping them in sync.

Samba can use standard Linux users, but there’s no way to reuse the same passwords.