Whether you previously deleted columns and like to bring them back or new versions of the source data have new columns, it's very non user friendly to add such columns (having to go through previous Applied Steps). In the latter case, it might even be impossible.

Please consider making it easy and straightforward like in Text Import where all you have to do is click Edit on the query and run the wizard again.

Sources:
1) https://superuser.com/questions/1496163/how-to-add-previously-deleted-csv-columns-in-power-query/1496624
2) https://superuser.com/questions/1497174/how-to-add-new-csv-columns-in-power-query/1497727