Is it possible to add files to a compressed archive without repacking?

Question

Or do I allways have to unpack, add new files, compress again?

Answer 1

That depends on the type of compressed archive.

If it's an archive of files that is subsequently compressed (like a gzipped tar file) then you will have to decompress, add the file(s) and recompress.

If it's a compressed archive format (like ZIP or RAR) then they generally have an option to add files to the archive without unpacking the entire archive first.