// For test purposes, start by creating (or re-creating) the data file
put {{
C1,C2,C3,C4,C5
1,5,9,13,17
2,6,10,14,18
3,7,11,15,19
4,8,12,16,20
}} into file "myData.csv"

// Read the file as a list of lists (rather than as the default list of property lists)
put CSVValue(file "myData.csv", asLists:Yes) into csvData

insert "SUM" into item 1 of csvData -- add a new column heading

// Go through all of the data rows to add the sum
repeat with rowNum= 2 to the number of items in csvData
	insert the sum of item rowNum of csvData into item rowNum of csvData
end repeat

put csvData -- see the modified data as a list of lists

put CSVFormat of csvData into file "myData.csv"

put file "myData.csv" -- display the updated file contents