Bulk Load (Copy) - First n columns.

Hello, I have a table with 300+ columns and I am using COPY command to load the file to the table. The issue I have is the feed or the csv file I have may only 1st 100 column values or 1st n column values. How do I let COPY command to know if the feed does not have values for those columns insert NULL values and load the rest without failing ?.