One of the most reason of becoming wordpress is as a powerful CMS is its action and filter hooks. One can make modifications in any part of site (both front-end and back-end) using these hooks.
You can also simply add new columns to the users list of wp-admin by using two great action and filter hooks of wordpress i.e. manage_users_columns (filter hook) and manage_users_custom_column (action hook). Filter manage_users_columns is used to add heading/label of new columns in header of users table and action hook manage_users_custom_column is used to show values of new field for each user in the list.