well... I'm on linux here so i can't simply "install" sm files, I can move them to the packages folder or I can do something else.....
the good thing here is, linux does NOT determine a file type by it's extension, so a file named virus.mp3 will be identified as a executable (where windows thinks it's an mp3 because it has the mp3 extension)
same thing for smzip files, on my pc they show up as regular zip-files, and I can easily extract them, usually It contains a songs folder, a group-subfolder and several subfolders for the songs.
just dump the songfolders in a single folder and place that folder inside your stepmania5/songs.
so in the end you'll get:
stepmania 5/Songs/group folder/song folders/
just use winzip or winrar or any other tool to extract smzip files :-)