Web Designer is not a HTML editor, so it cannot be used to open your existing website created in Frontpage, Dreamweaver or any other web authoring tool.
The Xara programs do have an Import from Web feature which will import the images, graphics and text from a web page and lay it out within the program as it's seen in a web browser, but this in no way replaces editing your website in the manner you currently edit it in.
This will not import or convert any objects (graphics, nav bar, button etc) so they are native, editable Web Designer objects, they will all be individual graphics.
No mouse over effects will be imported and neither will any links.
If you have a PSD file of your website then it is possible to import the PSD in to Xara.