You'll want to use a file containing the following information:
Then run the command:
cat passwords.txt | chpasswd
Don't forget to delete your passwords.txt file just after you're done. Seriously bad idea to leave this file hanging around.
If you're looking for a way to automatically generate a whole bunch of passwords that your users won't balk at, the pwgen command may be installed on your system already!
If not, and you don't want to go hunting for one, try this: http://www.multicians.org/thvv/gpw.html