It should work-Poser dropped absolute paths long ago.  You could just copy the runtme over and add it to as an external runtime, but what I would do, is to copy the Windows Poser Runtime to the Poser installation path on the Mac, then install Poser.  The Poser installation will then overwrite the files that need to be overwritten.

If you have already installed Poser, you could copy the Mac Runtime to another folder, copy the Windows Runtime into your Poser folder, allowing it to overwrite, then copy the Mac copy back, which will overwrite what needs to be overwritten.

The only issue might be file name/directory case, but Windows does preserve case, even though it isn't case sensitive itself...

EDIT I just reread esther's post above, and I didn't realise that the OSX  GUI didn't merge when copying.  (I'm familiar with UNIX, which is what OS X is under that pretty GUI) You could use the 'cp' command in a command shell, which does merge.  If you open the shell in the folder containing the Runtime folder; cp -r -f /Runtime '/path to poser folder'

More info here:  http://unixhelp.ed.ac.uk/CGI/man-cgi?cp

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