The DAZ installers themselves will correctly merge new files into an existing Poser Runtime without replacing any folders already there. In other words, the DAZ installers will not delete any existing content in your target library.

It's only if you move a folder in Finder (the Mac equivalent of Windows Explorer) that it will replace an existing folder of the same name (and all its contents). Like Explorer, it prompts you before doing this, but unlike Explorer there is no option to merge folder contents.

There is a point during the DAZ install where you can select the target location for the new files. In the older style DAZ installer for Mac it's easy to miss this option, and it will just use the default location. When specifying the target location, choose the parent folder of the Runtime folder you want the product to go into. This folder name will be the name of the Library as seen within Poser.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)