Save the file as .htm. All FP or Expression Web can work with are the HTML
code and the flattened images to put into it really. No web editor will
handle a Publisher File so you'll have to export everything just as if you
were having publisher create your final page.
There's a problem though, Publisher creates horrible, horrible HTML code.
Like similar tools, Publisher is designed for desktop publishing and not for
web design. Hence, it produces horrible code just like the similar tools do.
You may end up re-designing a lot of things in EW or FP anyways. You'll also
want to check your publisher site in other browsers because there's a good
chance it isn't going to be nice.