While I haven't had the need to use this yet myself, I 
realized that the free program Delegate 
(www.delegate.org) can translate https to http and 
vice versa.  It is mentioned in the features list:
<http://www.delegate.org/delegate/features/>
"non-HTTPS client to HTTPS server, HTTPS client to non-
HTTPS server"

As far as I can tell, WinHTTrack qualifies as a non-
HTTPS client, and thus Delegate could be used as an 
intermediary between it and the HTTPS server.

I can't guarantee it will work, but until HTTrack has 
HTTPS support built in, this may be your best bet.

BTW, Delegate has many other uses (creating HTTPS 
tunnels, POP3 tunnels, etc) but this specific use 
seemed most relevent to WinHTTrack users.

Note to Xavier...this program is free and open 
source...the copyright terms are simple:
<http://www.delegate.org/delegate/download/> (use FTP 
gateway portion)
<http://www.delegate.org/anonftp/DeleGate/COPYRIGHT>
/*/////////////////////////////////////////////////////
///////////////////
Copyright (c) 1994-2000 Yutaka Sato and ETL,AIST,MITI
Copyright (c) 2001-2002 National Institute of Advanced 
Industrial Science and Technology (AIST)

Permission to use, copy, and distribute (via 
publically accessible
on-line media) this material for any purpose and 
without fee is hereby
granted, provided that the above copyright notice and 
this permission
notice appear in all copies.
AIST MAKES NO REPRESENTATIONS ABOUT THE ACCURACY OR 
SUITABILITY OF THIS
MATERIAL FOR ANY PURPOSE.  IT IS PROVIDED "AS IS", 
WITHOUT ANY EXPRESS
OR IMPLIED WARRANTIES.
///////////////////////////////////////////////////////
/////////////////*/

According to the HTTrack credits page all of HTTrack 
except the java class parser by Yann Philippot is 
Xavier's work, which he has released under the GPL.  I 
think that means if both he and Yann Philippot want 
to, they can re-release newer HTTracks under non-GPL 
licenses, or under both GPL and non-GPL licenses.  As 
the copyright owner of HTTrack, I believe Xavier can 
switch his program off of GPL if he so chose (and had 
permission from Yann and any other people whose code 
he's incorporated...)

Benefit of going non-GPL (to something similar to the 
MPL or whatever) is you can incorporate other open 
source yet non-GPL code into the program.  I think GPL 
prevents non-GPL code from being used in GPL licensed 
programs (hence the concept of GPL incompatible, yet 
still open-source...)