> Is there a way to modify the HTTP-Accept header ?
No - exept by using the "send-header" callback (see 
<http://www.httrack.com/html/plug.html>) - the "buff" char* 
buffer argument can be modified up to 8KB to do that.

> Winhttrack doesn't claims to accept application/xml+xhtml
> MIME Type

That's right - but HTTrack won't be able to parse the 
application/xml+xhtml type, as it is not yet "seen" as an 
html type anyway.