I'm new here and just looking at how I would import a live website into the application so that I can edit and modify it.
Insert a custom component in to a new page design and then copy and past me your html in to that. Then create a new style sheet and do same I guess
Not sure how it works with previous sites, but I guess you can try it.
The app has an Import HTML function if you right click the Pages section header of the Design area on the right bottom area of the app. I just tried it with a blank HTML page that I had sitting around easily accessible and it worked to open that page in the visual area.
Make a new design project and import your pages and your CSS files and JS files etc. and see what happens. Post again and let us know if it works for you as I'm curious myself to see if it works but am too busy at the moment to find time to mess with it all. :)