You might need the combination of two modules: excel importer and the excel exporter. Follow the documentation for the following:
I agree with Thijs that this can be achieved with the Excel Importer and perhaps the CSV Exporter module. This is the best option, but does come with a fair share of overhead.
If you don’t really need the data in the Mendix app, but really only want to automate then a simple Java action could do the trick. If the files are uploaded in Mendix, create a Java action that gets a CSV file and an Excel file as input and reads the content of this input Excel file and appends the content to this existing CSV file. Call this Java action as many times as you have Excel file content to append to the CSV file. Should be fairly straightforward if you or your team mates have basic Java skills.