I'm in a similar situation and considered doing it using custom fields, but it's easier to leverage the built-in attributes of users (mostly I needed author archive pages). Basically did what @wphelpcenter said, setting up accounts using the same password and administrative email. This works pretty well for creating multiple accounts with the same email address: http://coffee2code.com/wp-plugins/allow-multiple-accounts/
So far as I can tell itâ€™s working great on my installation of 2.8.6. Only problem I ran into was a multiple email address error when trying to edit usersâ€™ accounts after theyâ€™d been created (to change roles, etc.) Fixed easily by commenting out lines 177-178 of /wp-admin/includes/user.php
I'm also using http://www.dagondesign.com/articles/import-users-plugin-for-wordpress/ to import users in bulk, but same deal, have to get rid of lines 176-180 to stop the email address check.
Now I'm looking for a better way to pick authors when posting, preferably an autocomplete text box...the drop-down on the post page isn't cutting it. http://wordpress.org/extend/plugins/co-authors-plus/installation/ seems to be working so far...