You should be able to add a FTP user via this command:
sudo useradd userftp -p your_password -d /home/FTP-shared -s /bin/false
userftp is the username,
your_password is the password, and
/home/FTP-Shared is the path.
I personally use this in
proftpd, but I'm pretty sure the command should be universal.
Note that this user has been created only for FTP access, and as a result does not need a valid shell (making it slightly more secure) hence the "bin/false".
So basically for your case, just create the user and set its home directory to
/var/www. However, remember to first
chmod the directory appropriately so that the new user may access it (be sure to add him to the appropriate group, etc).