Here how have make it to work:
in the 
/packages/acs-tcl/request-processor-procs.tcl
proc 
rp_handler
....
  ad_call_proc_if_exists ds_add rp [list transformation [list notfound $root/$path notfound] $startclicks [clock clicks]]
  #ns_returnnotfound
  rp_serve_abstract_file [acs_root_dir]/www/global/notfound.adp