>   There is a problem downloading big files with HTTrack
> because it is not really download manager and cannot
> continue broken downloads (can only restart in next
> session)

Well, it will resume during the next "update" 
or "continue", yes.

> How can I tell HTTrack to use external program for
> downloading files of some types (or bigger than some size)
> instead download by itself?
This isn't possible - but broken files should not occur 
very often anyway? (except with overloaded servers, or 
network problems?)