If I were you, I would just manually copy the files or Runtime from your (I can't remember the exact path the old OS 9 installer used) OS9 Applications folder where Poser Pro Pack was originally installed, to your new Runtime in your Applications/Poser6/Runtime folder.  The problem is that DAZ never updated the old installers to use an OSX compatible installer. 

Also, you could try just adding a new Runtime in Poser6 and point it to your old Poser Pro Pack runtime.

Also, if you can run your Mac in Classic Mode, you could always just change the file path in the installer to install it in your Applications/Poser6/ folder instead of the default path.

Just some ideas.:)

-hOss