If you create a named range in the Excel spreadsheet, you can use it as a table name in an Excel connection and insert directly into that part of the spreadsheet.
If the SSIS package runs on a system with Microsoft Office installed, you could use object automation in a script task to open the spreadsheet and use Excel methods to populate specific cells.
PowerShell also can do a great job of working with a spreadsheet. Assuming also is running on a system with Excel available.