Make sure you have 'Pay Later' Column set to 1 and Status column set to 'Pending' for the contribution rows that are pending from pay later. If you don't have status column that its set as Completed by default
Maybe, the easiest way is, to open the csv in Excel (or LibreCalc) and do some magic with formulas?
Or even much easier: Open the file in Excel (or LibreCalc) and do some search and replace in the column?
That's the way I would do it, if the file isn't to big for Excel and it isn't a work I had to do every day.