If you imported customers from Zencart and the hash of password was produced by this function:

// from Zencart version 3.8
function zen_encrypt_password($plain) {
$password = '';

for ($i=0; $i<10; $i++) {
$password .= zen_rand();
}

$salt = substr(md5($password), 0, 2);

$password = md5($salt . $plain) . ':' . $salt;

return $password;
}