Wouldn't it be convenient to just link the runtime as an external rather than merge?  Merging Poser runtimes is taboo especially from earlier to current versions.  Internal scripts will become completely corrupted and unusable since scripting has changed since P4.

The most involved method but most secure, open each file folder and copy content folders into their appropriate locations.  This avoids overwriting any particular section and keeps everything intact.  Now, I do believe there is unzipping programs which will do exactly what you want with new content.  But I can't provide any further help because I'm strictly PC.