I tried it myself with WinZip. From Windows Exploer I right
clicked on a pzz file test.pzz and used the Open With option.
Then I chose WinZip to open it. WinZip had a pop-up asking me
to give the full name of the file extension and I entered test.pz3.
Then there in Winzip there was the uncompressed file and I put
it in a directory.
Then I opened the file with Poser 6 and it looked just fine.