> Our website has secured pages which only logged in users 
> can get to. But there's a way to get to those pages with 
> the testuser/password by providing some special query 
> string.

httrack is normally able to hold cookies and replay them
do you have any cookies.txt file being generated by httrack?
> One more question, is there any way that I can put the 
> user/password to get to secured pages with this tool?
If the authentication is a standard (that is, http-based 
basic authentication and not form-based one) you might try
<http://user:pass@www.example.com/whatever/>