Hi @mlefler,
You could certainly build an app with a file input to take cxv/excel sheets and append that data to your database 
You could build out the logic to parse the file they input and append it. If your user has a paid seat in your Retool org, they could log on to the app and submit the new sheet
You could also create a public Retool Form (similar example here), but you wouldn't have the benefit of authenticating your user
Keep us posted if you run into any blockers or questions