It already is.
You can download bundles for the offline version from here:
https://spnati.net/bundles/download-bundles.html
Follow the instructions on that page to download and set up the offline version. This will allow you to download the entire game and all its files; note that this is a several-GB download thanks to all the character sprites!
Extract all the files, and you'll have offline SPNatI. Opening config.xml in the main folder (use Notepad) will allow you to change some settings, such as enabling debug mode and unlocking all endings. Open index.html in your browser to play the game