There is no way to import "any" type of template other than a CSS template. Any themes/templates you purchase from websites selling them cannot be used other than the supporting CSS, JS and image files. You will always have to recreate the website's HTML structure within BSS.
Let me add on to that, as no way to import HTML without it becoming a full Custom Code block. No drag and drop or settings will work, all will need to be done manually which would be a whole lot easier in a text editor than in BSS.