First, you must find the extension's ID. To do this, go to chrome://extensions and click on the extension of choice. Then look for the ID and copy it.
If the ID doesn't show up like in the image above, you can also find it in the URL.
Once you have the extension's ID, open the following link: chrome-extension://id-here/manifest.json (replace "id-here" with the ID you got earlier).
Once you are on the page, you need to find a web-accessible resource with an explicit file name. These are typically near the bottom of the page. Look for something like this:
You can now copy any of the resources and replace the manifest.json part of the URL with your resource. Then go to the URL and confirm that it works. If it doesn’t, you’ll have to go back to step 2 and use a different resource.
Congrats! You can now enable developer mode from settings and add a temporary extension. Use this web-accessible resource URL when prompted.
You can also make a pull request on this repository for it to be added permanently.