Sure. The Greenworks library lets any JavaScript game integrate with Steamworks in NW.js or Electron: https://github.com/greenheartgames/greenworks

It is; here's a very brief "how to".

http://www.andy-howard.com/phaser-to-steam/

You can publish NWJS/Electron apps for steam. You can plug a Phaser game into an NWJS/Electron app.

So yes.