I am guessing it isn’t so much the Google Script itself, but rather that the firewall requires some kind of SSO token in the REST calls being made by the script. You won’t be able to convert the spreadsheet to XLS as is because there is a ton of Google App Script behind it. GAS is basically an old version of Javascript. I know Excel now has some support for Javascript custom functions, but the underlying framework Google Sheets uses is quite different than Excel. It would be a lot of work. Creating an Excel version is something I have given thought to, but that would be down the road a bit.