Thanks for your post. You can use the ‘Create Zip File’ action and set the Zip Action to ‘Update’, then point it at the existing zip file and tell it to add file c.text. The action should only update that one file, leaving the others alone.
We’ve found that the ‘Create Zip File’ action can be a bit fussy about zip files that it did not originally create. Was the file originally created by FinalBuilder?
If not, is it too much trouble to unzip the files and then rezip them using the FinalBuilder action?
Alternatively, if you install the free 7Zip then you can use the “Update Archive” action from the 7Zip package to update normal zip files, as well as 7z files.