It's easy actually. Run "System Save" action, then in "On Save complete" event expression SaveStateJSON will contain all saved data. To save it to disk with any extension use "Browser request download of string" or "NWJS Write text file".
To load - read the file from disk into a string variable and use "System Load from JSON".
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It's also possible to compress data (as files in JSON format may be pretty large). See this plugin. And again, you don't need to use "zip" extension for compressed files, you can choose any extension you like.