FreeBSD 8.3 master.passwd to 10 ?

I don't think there's anything to "convert". The syntax of the file hasn't changed for decades. But make sure you read pwd_mkdb(8) as the authentication is actually done from /etc/spwd.db and /etc/pwd.db.

     Regenerate the password database after manually editing or replacing the
     password file:

            /usr/sbin/pwd_mkdb -p /etc/master.passwd
It should work as Sirdice says. If you want to use the updated password hashes (sha256 vs MD5 - which is still supported), you'll have to re-enter the respective passwords after the update.