Currently I donot see a perfect solution. But something you could try is to export all the pages as separate Fireworks png files. Then import each file in a new page.
This should give you a large png file with all the pages in it. The downsize will be that the attributes like Master page layers, Shared layer across pages, symbols will loose its information.
The suggested method above can be used for aggregation at end and such a file will not help if edited directly.
another way is create a new page with a bigger canvas and share those 2 pages layers to the new page.
of course it also depends on your working files
Sarthak's suggestion is going to help you and it's quite easy.
Start a new document, or open an existing one, create a new page and import a png file. And then, the same for as many pages you want to import.
With CS5, you can just save a png with multiple pages and then import each one of them, into a document, by using the import option.
You don't have to create new pages, just select to import a png, then at the import menu that appears, select insert after current page and FW will automatically add a new page.