It already has the JSON importer to get the current rates. It wouldn't be too difficult to take a snapshot and save that off. I've modified my personal copy to have a button to refresh the rates and another button to take a snapshot of my holdings and save them off so I can see the change over time. I don't want to add those to the main spreadsheet though because once I added the button it asked me give the spreadsheet permissions to my account which I'm guessing most people will not prefer.