From the Data Campaigns page, find and click on the CSV campaign that you would like to add data to. This will open up the campaign details page. Click the 'Add Enrichment Data' button on the upper left-hand side of the page. On the 'Upload Your File' modal, drop or browse for the CSV file that you would like to upload.
Note: The columns on the additional CSV file to be uploaded must match the columns of the original CSV file used during the campaign creation.