Ruling out maidump, which used folders, you are talking about Vitamin only.
You can install folders by pressing a button in Vitashell, I don't see how this doesn't mean installable. PSP ISO files are different, because the iso format actually possess an image like structure, while .zip files do not contain any additional structure or functionality.
I didn't try, but it could be possible to pack them. It could create problems, but it's possible in theory. Think that you can also do the opposite, converting a .vpk dump to folder-format (maidump).