Hi jenkinchu, thanks for asking, just to clarify I assume you want to add a custom User field that appears in the Order export fields list? Is this correct?
To do this simply add the custom User meta name to the Custom User Meta textbox under WooCommerce > Store Export > Quick Export > Export Type: Users > Custom User Fields section. If you want to add a non-User meta field (e.g. a column from the wp_users table) then you’ll need to use our exposed WordPress Filters, here’s a resource:
Hope this helps! 🙂