Add a directory ~/some-other-directory to an existing files.tar archive.
You can use this short tar command.
              
                  tar -rvf files.tar ~/some-other-directory-somewhere
              
          
      Or this more verbose tar command.
              
                  tar --append --verbose --file files.tar ~/some-other-directory-somewhere
              
          
      You can append multiple directories onto an exsiting tar archive by simply chaining the directories.
              
                  tar --append --verbose --file files.tar ~/some-other-directory-somewhere ~/another-directory-somewhere
              
          
      -r or --append tells tar to append to an existing archivev or --verbosetells tar to produce verbose outputf or --filetells tar that the next parameter will be the tar file to create, extract, append to, list, etc.