A few months ago I looked for a mac utility that could expand files with the extension .z7 (or .7z) but couldn't find any, but I found one now so I'm putting it in the forum database so when people search for it in the future they'll find this answer. It also decompresses Zip, Tar-GZip, Tar-BZip2, Rar, 7-zip (7zip), LhA, and many other obscure formats. The program is called The Unarchiver, and it's free and open source. You can basically get rid of stuffit expander. Check it out.
http://wakaba.c3.cx/s/apps/unarchiver.html
http://wakaba.c3.cx/s/apps/unarchiver.html