Right-click on Starsector.app, select show package contents, there are mods and saves folders inside just like you'd see on other platforms - you should be able to copy things in just fine. (I don't personally recommend 7zip over just using the standard .zip format, but whatever works for you - the important part is that you get the files moved, not what format they're in when you're moving them. Just remember to decompress them again at the destination.)
The vmparams file is a bit different, though; you'll want to edit Contents/MacOS/starsector_mac.sh - or, at least, that's how I've always made my changes to vmparams.