RAR != ZIP.

Fell free to read a stack overflow about that situation

Your choices are basically “rewrite the entire file” or “leave the original file in place, do an append and try and hide the old file.”

Editing old data in most streaming file formats with inline metadata is basically impossible because they compact the data as much as possible and internally refer to offsets.

Appending is trivial, editing is very hard if not downright impossible.