If it's a classic Nintex Form, you could use JavaScript to do this. But your description/question is a bit..... awkward. If I understand you correctly, you want to fill in Nintex Forms and add the data as a new row to an Excel file in another SharePoint library. If the Nintex Form is on a SharePoint list, you could export this list (or actual the current view) to Excel.....
Yes you are correct that my question is awkward, sorry and you summarized what I am trying to do accurately. I'm glad there is a solution. Is there a quick way to know what Javascript I would need to do what you suggested?