> I already have one big file downloaded from the site

> place and I want HTTrack to see that file as
> downloaded so that if I update the mirror, it's as
> if HTTrack downloaded that file

If you already have a mirror, then HTT should have already downloaded the
file. It won't redownload on update (unless you used cancel or killed the
program because continue is broken.)

If you don't have a mirror, HTT won't know about the file and will mirror it
again. You can add a filter to prevent that and after each Update copy the
file back and change html pages to point to it.