He meant that the ePub file is actually a zip archive (ie. container). You can open it with WinRAR, 7-zip, etc. and add that file to the what's-his-face
folder, as long as:
"The first file in the ZIP Container MUST be a file by the ASCII name of ‘mimetype’ which holds the MIME type for the ZIP Container (i.e., “application/epub+zip” as a 20 character ASCII string; no padding, CR/LF, white-space or case change). The file MUST NOT be compressed nor encrypted and there MUST NOT be an extra field in its ZIP header. "
...meaning don't extract the entire archive and re-zip it back the way it was if you don't know how to add something uncompressed.
This is only for iBooks. Other readers probably just ignore that file, am I right?