This post refers to the on premise SharePoint. In O365, UNLESS you are using a CSV formatted file, you are not able to anyhow update excel file contents.
You could try to generate Excel file (using the action), however this allows you only to create a new file out of the existing template, so you cannot take an existing file and add a record to it. Sorry
In on premise you have an access to the (but only in both SharePoint Enterprise and Nintex Enterprise versions), that does allow you to do what you are asking for. In O365 you are very limited when talking about office documents manipulation.