Generally, no. You will be unable to import an entire site verbatim, because Squarespace operates by managing your content. Tools like Dreamweaver and FrontPage produce pages that have your content organized in no particular fashion, and thus we can not extract the information required to produce the equivalent Squarespace structures.
We strongly recommend using the built in Squarespace editor to manipulate your pages. If you develop code using an outside tool and wish to import the HTML content, you can paste this HTML directly into a Squarespace Page that's set to accept Raw HTML from the editor screen. We don't recommend this, as the steps involved are far too many (which is why we have built WYSIWYG editors for use on the web).