Using John the Ripper with custom encryption function

Question

I have a working script done with python to do dictionary attack on a few hashes

import hashlib

def main():
    print("Starting...")

    users = []
    with open('users.txt') as f:
        for line in f:
            splitted = line.split()
            users.append(User(splitted[1], splitted[2], splitted[3]))

    with open("hashes.txt", 'w') as f:
        for user in users:
            f.write(user.hash + '\n')

    for user in users:
        print(user.name, user.salt, user.hash)
    print()
    i = 0
    with open("10-million-password-list-top-1000000.txt") as f:
        for pwd in f:
            pw = pwd.rstrip()
            i += 1
            if i % 100000 == 0:
                # Still running
                print(i)
            for user in users:
                newHash = get32Hash(pw, user.salt)
                if newHash == user.hash:
                    print("Password found: " + pw + " for user "+ user.name)
                    with open('crakked.txt', 'a') as f:
                        f.write(user.name + ' : ' + pw + '\n')
                    users.remove(user)

def get32Hash(pwd, salt):
    return hashlib.sha256(b"[generic_salt]" + str.encode(pwd) + str.encode(user_salt)).hexdigest()[0:32]

class User:
    def __init__(self, name, salt, _hash):
        self.name = name
        self.salt = salt
        self.hash = _hash

main()

All the hashes and their custom salts are marked in users.txt

How can I use John-the-Ripper with the custom limited 32-bit length hash from get32Hash-function? It would be much more efficient to be able to use mangling etc. from John.