I am a relatively new Mac user (just a few months) and have searched for but not found any evidence that this is possible on a Mac via the built-in finder. There is a command line utility that will do it, ditto, but that sort of thing isn't for everybody.

I did find this free tool, but haven't tried it.

http://mac.softpedia.com/get/System-Utilities/Folder-Merge.shtml

Just a note to clarify this for new users: Poser does not install content for you. You're expected to manage files yourself, via whatever OS-supplied or add-on tool your personality and budget demand.

Poser let's you activate an external runtime library by "adding" it in the Poser library GUI. But it must already exist.

Poser does provide one mechanism to automate the install-merge of a zip file. It's in the "Content" room, but it only installs to the pre-built "Downloads" runtime library. It does not ask you where you want the stuff to go. I have no idea why.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)