I don't know any way of doing this with the standard 'ftp' command, since it cant exactly send files to stdout.
I don't know anything about kerberos tickets either, so I can't help you with that.
I do believe, however, that you can
get wget to connect with a username and password by specifying them in the url thus:
wget -O - "ftp://username:firstname.lastname@example.org/backup.tar.gz" | tar -xvzf -