Each "column" in your CSV file is going to correspond to a different field in the FileMaker table. You cannot automatically add new fields to the table. Each time you add a new column to your CSV file, you'd need to add a new field and then update the import records step in the script to map that new column to the new field.
Import Records has a lot of different options and we can't see those options from here. You might want to start by looking up this tool in FileMaker help so you can figure out the basics of this operation, then open up the script step to see what options were used in the script. Feel free to post back here with more questions if needed.