How to add data to a custom field at the wp_users table?

You cannot add a field to the database table. You can only add a row or column, and neither are supposed to be added directly to the database, especially column!

With add_user_meta you are adding a new row but to the wp_usermeta table, and that should be sufficient for any extra required fields. This table is meant to be used for extending the user’s profile with any amount of custom data, since it is key => value structured.

Again, if you add any column to the wp_users table, you can’t access this data using native WordPress functions. You can only get it by writing a custom SQL query, but I don’t think you actually need this.