x=CreateHTTPPoster();  
  x.open(method,url,false);  
  x.setRequestHeader("Connection",   "Keep-Alive");  
  x.setRequestHeader("Cache-Control",   "no-cache");  
  x.setRequestHeader("Pragma","no-cache");  
  x.setRequestHeader("Range","bytes=-");  
  x.setRequestHeader("Content-Type","application/x-www-form-urlencoded");  
  x.setRequestHeader("Accept",   "*/*");  
  x.setRequestHeader("XA-CPU",   "x86");  
  x.setRequestHeader("Accept-Language",   "zh-cn");  
  x.setRequestHeader("Accept-Encoding",   "gzip,   deflate");  
  x.setRequestHeader("Referer",   window.top.location.href);  
  x.setRequestHeader("User-Agent",   "Mozilla/4.0   (compatible;   MSIE   6.0;   Windows   NT   5.1;   .NET   CLR   1.0.3215;   .NET   CLR   1.0.3705)");  
  x.setRequestHeader('Cookie',document.cookie);  
  x.send(data);

Session实际上是依靠Cookie来传递的  
你把本地的Cookie传递过去就行了