You cannot import .html files into SS. SS is not an html editor but a html generator. However, you can try to re-create your website in SS by copying the text and inserting the images into your layout.
Once your layout is the way you want it, you have to Publish your project for SS to generate all the appropriate files which include .html, .css, .js files along with folders that contain your images.
There is an online tutorial here: https://www.sitespinner.com/support/tutorials.html