Can I use Excel as a data source in retool?
Not natively, but I know excel has a REST API and you can connect to almost any REST API through Retool. it's a somewhat complicated setup, but I was able to find this video on some of the necessary setup/ functionality. It sounds like these endpoints use OAuth2.0, link to our docs on this here. It doesn't look like many of their docs on this have been updated since 2017, so again, it might be a little complicated to venture into.
You could export your excel file as a CSV and use the filepicker component to import it directly! We also have a Google Sheets integration if that would be helpful at all (Retool docs here).