Rated 5 out of 5 stars
Yes, I did the same thing. An xpi is actually recognized by WinRar as a .rar. I don't know if WinZiip will open it, but WinRar does (add the .xpi file association to WinRar, or browse with WinRar to the xpi and open it)
Making the manifest file worked for me. I'm surprised the developer hasn't posted this yet. It's such an easy change.