You have the ability to add additional fields in your custom tables. Instead of doing it one at a time using the Add Field function, you can import a .CSV file with several in one file.
Once you click on Re-Upload File, you will see this pop-up.
As you can tell, new fields were added to the bottom of my table.
Keep in mind, this feature does not change the order of your fields nor does it overwrite existing fields. This only adds additional fields that were not already there.