The quickest way to add a column to an excel file with already-existing, clearly defined rows is to iterate once through the rows and add a column at the end of each iteration using the below code.
FileInputStream excelFile = new FileInputStream(new File(fileDirectory+file));
Workbook workbook = new XSSFWorkbook(excelFile);
Sheet datatypeSheet = workbook.getSheetAt(0);
Iterator<Row> iterator = datatypeSheet.iterator();
// Add additional column for results
while (iterator.hasNext()) {
Row currentRow = iterator.next();
Cell cell = currentRow.createCell(currentRow.getLastCellNum(), CellType.STRING);
if(currentRow.getRowNum() == 0)
cell.setCellValue("NEW-COLUMN");
}