Very roughly:
# cp /etc/passwd /etc/passwd.bak # just in case you e.g. forget one of the >s
# echo 'ftpuser:x:1000:1000:ftp user:/home/ftpuser:/bin/sh' >>/etc/passwd
# cp /etc/shadow /etc/shadow.bak
# echo 'ftpuser:x:15020:0:99999:7:::' >>/etc/shadow
# cp /etc/group /etc/group.bak
# echo 'ftpuser:x:1000:' >>/etc/group
# cp /etc/gshadow /etc/gshadow.bak
# echo 'ftpuser:x:1000:' >>/etc/gshadow
# passwd ftpuser
# mkdir /home/ftpuser
# chown ftpuser:ftpuser /home/ftpuser
You should of course make sure the username, user id, and group id are available. Also, unless the account needs shell access for some reason, set the shell to something like /bin/false or /sbin/nologin (if the latter is available).