> I understand that there is no way to continue
> downloading previously downloaded files (original
> files downloaded via wget) is not through httrack?
Correct.  Httrack does not know about files it hasn't tried to download and
will download them again.

If there is a pattern to what you already have you can filter out those.  You
have files download\aaaaa through download\cxxx then add a filter
-*\download\*[a-c]*