Heres a note from Dave Ritter <firstname.lastname@example.org> in regard to importing sites generated by Microsoft FrontPage:
Front Page extensions on the remote site should not cause a problem since Fusion simply imports HTML pages, as it understands or interprests the various page relationships. FP98 manages a web of individual pages; it also includes various files to keep track of changes. Fusion, at best, would only check the HTML pages, non of the other stuff that FP manages. Fusion, on the other hand, maintains a single file for the entire web and generates the HTML pages only at publish time; the html pages are never edited by Fusion.
Since Fusion will analyze the FP web pages (not the FP web), it is extremely important that you have a well-structured web site in FP. Fusion forces you to maintain a hierarchy-structured site; in FP it is optional. So, depending on how you have the hyperlinks between the pages for FP, the structure may not be recognizable to Fusion.
To have the best chance of importing, make sure you use the FP98 navigation view to develop a hierarchy structure as you want the site to be imported. Make sure tha the only hyperlinks you have between your HTML pages are created with the automatic Nav bars in FP98; otherwise you may have hyperlinks pointing up and down the hierarchy structure. You can get an idea about how the structure will be recognized by adding a FP98 contents page; if FP shows the site outline correctly, it is likely that Fusion will also.
FP will allow multiple top-level pages; Fusion will not. So, make sure you only have on top level page before attempting to import the pages.
Another point, make sure you remove any FP98 components which require FP extensions; the will only confuse Fusion. This includes counters and search components.