I use Homebrew for package management, so the instructions here work with that. You could probably get p7zip and atool another way if you want.

1. install p7zip with homebrew: `brew install p7zip`
1. install atool with homebrew: `brew install atool`
1. extract individual files from .7z archives to current directory: `atool -X . -e -E *.7z`
1. delete or move the .7z files: `rm *.7z`
1. archive each file into its own ZIP archive using `find` and `atool`: `find . -d 1 -type f -exec atool -a -F 7z -E -e {} ';'`
1. delete the uncompressed rom files. They should have a common file extension, which you'll need to change depending on the ROM type. For example, I needed to delete a bunch of .gb and .gbc files: `rm *.gb; rm *.gbc`