Hi Melanie,
You could build an integration from a Mendix app to Excel online so that you could exchange data between your spreadsheet and your Mendix app. That would be a reasonable amount of effort.
For much less effort, you can import the Excel spreadsheet into your app and use the data to populate your Mendix app.
Not sure which of these is closer to what you want to do. If you can provide a bit more detail?
Thanks,
Mike