What you describe is generally considered poor practice in the world of relational databases. It is also possible that your current data design is poor if this sort of thing, adding variables to the same-named table, is needed.
A more typical approach would be to have a view or query that reads the existing data table that had the results needed and use the view for reporting or whatever that table with the additions is used for. Properly designed views would use "new" table after import or update or what ever is causing new data. Then nothing to delete, just remember to use the view instead of the raw data table.