One of the features of XSitePro is that you can create web pages very quickly.
This is possible because XSitePro breaks up the page in to section (header, footer, side panels and main panel).
This allows you to just focus on creating the section of the page that do not change much once.

The downside of this is that it is very difficult the import a published web-page and automatically split the page in to sections.

It is possible to recreate your existing website in XSitePro.
A published website is made up of standard website files, as these files are not in XSitePro format, it is impossible to import them as if it was a XSitePro export file or a backup.

What you can do however is recreate the design from the existing online website in XSitePro and then copy each page's content and paste it into newly created page.

You can save the images from you online website onto your hard drive a few different ways.
a) You can right click on the image on the website and select 'Save Image as...' (This may vary between different browsers)
b) You can type in your URL of your website and include '/images' at the end to view a directory listing of your images. For example 'http://www.yoursite.com/images'. Some servers do not allow directory listings.
c) You can access your server via a FTP client and browse to the images folder. A great FTP client you could use is 'Filezilla':
http://filezilla-project.org/