Sort of. You can paste content created in these systems into Squarespace Pages from the Architecture area of your site manager, which will be sufficient for most single pages. 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.
Can I import static HTML pages from Dreamweaver or Frontpage?