A Yahoo! .CSV file is a normally formatted .CSV file which:
Importing and exporting from Yahoo! Address Book using the Yahoo! .CSV file names gives you complete, direct access to the entire contact database you've built in your Yahoo! Address Book.
If you are comfortable importing data from an unsupported platform via .CSV file, we recommend you edit the .CSV file in a program like Microsoft Excel and change the field names to match exactly the Yahoo! field names listed earlier in field names for .CSV files. If your field names do not match the official Yahoo! field names exactly your import will be rejected. Be sure to save your Excel worksheet as a Windows .CSV file before trying to import it into your Yahoo! Address Book.