> is it possible to disable the replacement with "_"
> in build options?
I assume you mean the replacement of tilde "~" with underline "_"
<http://httrack.com/html/fcguide.html> states that that select characters (e.g.,
~, :, <, >, \, and @) are replaced by _ in all pathnames.

> I like to create files with "~". My Linux system has
> no problems with filenames like that.
> Example:
> www.domain.com/dir1/dir2/~filename.html

If you kept the site structure and it contained a .../~user/... url, and your
machine contained a login name "user", then the mirror would be broken under
most shells.

The other characters (:, <, >, and \) have to be replaced under windows.  The
tilde has special meaning to some programs under windows.

I don't know about Macs, nor the unicode conversions.

Since httrack runs on windows, mac, and unix it has to handle the least common
denominator.