Hey Mark, are you wanting to just append a line to a file on the local machine? If so, you can use a Script activity like below to append a line to a file. You can also create a Custom Script activity with this code if you need to use it in multiple places.
File file = new File("c:\\temp\\file.txt");
FileWriter writer = new FileWriter(file, true);
writer.write("new line in file\n");
The highlighted items can be replaced with your requirements and/or variables.