Adding data onto the end of a file

We're about to start a long galapagos run, and it would be nice to be able to check the data outside of galapagos while it's running generations. I've looked into GHOWL as a way of exporting the fitness values to a spreadsheet, but it looks like GHOWL overwrites the excel file each time. Short of creating a new spreadsheet every time galapagos runs a genome, does anyone know a way of tacking data onto the end of a spreadsheet?