Doesn't what you wish to achieve defeat the whole
purpose of HTTrack as a mirroring tool?
Your nose is "/".
Your question is: How can you ensure that when you 
look in the mirror the reflection of your nose is "-"? 

Some obvious answers. Use a distorting mirror, 
which however defeats the purpose of a mirror.
Get surgery to change your nose from / to -. This
solution not only lets mirrors do their job, it contains
one clue to a solution to your problem.

Mirror site1.com's structure, then perform a search 
for "*.txt" on the directory: 
my_mirrored_sites/site1.com.

Next copy all the search results into a directory
of your choice.