Manually setting password hashes

Not sure if this is the right place to post this, but I've got a possibly strange question. I'd like to be able to manually set the password hash for users, and I'm not sure if the ReadyNAS OS 6 supports sha512 hashes, or if I'm doing something else wrong. I want to do this because I'd like a user's password to be the same on the ReadyNAS as it is on another Linux box, without having them have to set their passwords twice. I ssh'd in to the box, made /etc/shadow writable, and edited a user's entry to include a password hash created on a different machine (CentOS). It didn't work - I see "FAILED with error NT_STATUS_WRONG_PASSWORD" in the log files. But I noticed that the passwords in /etc/shadow start with a $1 prefix, indicating that they are MD5 hash, while the password I pasted starts with a $6, indicating it's a SHA512 hash. Looking in the /etc/pam.d/common-password file, it appears that SHA512 is the hash of choice for the ReadyNAS:

# Explanation of pam_unix options:
#
# The "sha512" option enables salted SHA512 passwords.  Without this option,
# the default is Unix crypt.  Prior releases used the option "md5".
[...]
#password	[success=1 default=ignore]	pam_unix.so obscure sha512
password	[success=1 default=ignore]	pam_unix.so sha512

So I've got two questions:
First, why does it appear that the ReadyNAS is creating MD5 hashes instead of SHA512?

Second, why doesn't it seem to like my cut-and-pasted SHA512 password hashes created on another Linux box?

Thanks!