All commands tested on Mac OS Monterey / Sep 12, 2023

How to append a file to an existing tar archive

Add new-file.txt to the existing archive files.tar.

You can use this short tar command.

              
                  tar -rvf files.tar new-file.txt

Or this more verbose tar command.

              
                  tar --append --verbose --file files.tar new-file.txt

If you need to add multiple files to an exsiting archive, simply chain the filenames.

tar -rvf files.tar new-file.txt another-file.txt yet-another-file.txt

Options explained

-r or --append tells tar to append to an existing archive
v or --verbosetells tar to produce verbose output
f or --filetells tar that the next parameter will be the tar file to create, extract, append to, list, etc.