> Can Httrack put all external files in one local
> folder, and just rewrite all links in the html files
> that link to these files ?
Nope - there is no "external" links ; scan rules define the scope of
downloadable links, and httrack does not make any difference between a file
downloaded on a host A and a host B.

What you could do is to force all files to be inside the same first-level
directory structure using the "user-defined site structure" option, but this
won't do exactly what you want AFAIKS. (such as "www.mysite.com%p/%n%q.%t")