The Add Field function in ArcGIS Desktop allows users to add fields to a table. However, it is not possible to use this function to add a field in Microsoft Excel (.xls and .xlsx) and CSV (.csv) files as they cannot be edited and are set to read-only in ArcGIS. Attempts to add a field to these tables using the Add Field function fail and the function is displayed as inactive (grayed out). For more information, refer to Edit an active table.
The instructions provided describe how to export a CSV file to a geodatabase table in ArcGIS Desktop to add fields to the table.
Note: For Microsoft Excel files (.xls or .xlsx), change the extension of the file to .csv. This changes the table file format from an Excel file to a CSV file.