You really have two good options:

Set up a public FTP site and use the schedule data source feature in ServiceNow to load the CSV and have it transformed into a target table.

Use something like the Crossfuze File Importer (link provided by another comment on here on snguru.com) to simply have a remote CSV file sent to the SN instance via the mid server on a scheduled basis to ultimately be transformed by the system transform maps.