You can write one record with one column on the start position. It is a bit cumbersome but it works.
More convenient is the usage of the excel custom components from Talend Exchange.
There are dedicated components for this use case.
You need: tFileExcelWorkbookOpen, tFileExcelReferencedCellOutput and to write the file again: tFileExcelWorkbookSave.
https://exchange.talend.com/ search here for tFileExcel and you will get the whole component suite.