To update or append your mail table in BigQuery with new mail data each week, you can directly upload the new CSV file containing the updated data to BigQuery.
If I'm understanding your issue correctly, you are going to love external tables.
Basically you can define a table in BigQuery to read csv files in a GCS bucket. Then any time you add a csv file to the bucket, the table would reflect the new rows from that csv in the table.
Just be really careful to create your csv files in the exact same format.