As a matter of interest, I was testing a product for another vendor, and I tried placing the zip file next to an existing Runtime folder and extracting it. Results: it did not overwrite the existing Runtime folder and contents, nor merge into it - it created a new folder called Runtime-1 and put everything in there. Not much use.

This was using Stuffit Expander that comes with OS X 10.4 (Tiger).

"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)