The hosted web-browser based release of Twine 2 expects each of the installed story format's to also host their format.js files on a web-server.
As far as I know SugarCube has no official hosting site (besides Twine 2's) for it's compiled format.js files, and I am unsure what the developer's opinion would be of a non-official site hosting those format.js files publicly.
You could possible run a private web-server on your local machine and then host a private copy of the relevant format.js file on it, this would allow you to use a valid remote(ish) URL to add your private copy of the story format into the publicly hosted release of Twine 2.