Can I set a password for a member who has not previously registered with the site?

Technically, you can by doing this: Log out, then click on the Classmate Profiles list, click on the classmate's name, and join the site as the classmate, in which case you'd have to have an email for that person and you could set a password. I'd advise having the classmate's permission to do this.

Of course, once someone has joined the site, you can change their password.