type
  TThreadParams = packed record
   // Number: Integer;
    Dat: string;
  end;
  PThreadParams = ^TThreadParams;

function Umwandler(pDateiname: Pointer): Integer;
begin
... Umwandlung
end;

procedure TForm1.Button7Click(Sender: TObject);
var
  DateienLst: TSearchRec;
  id: Cardinal;
  ExitCode : Cardinal;
  tp : TThreadParams;
  threadhandle: THandle;
begin
  if FindFirst(Directory + '*.txt', faAnyFile and not faDirectory, DateienLst) = 0 then
    try
      repeat
        tp.Dat:= Directory+DateienLst.Name;
        threadhandle:= system.BeginThread(nil,0,@Umwandler,@tp,0,id);

        WaitForSingleObject(threadhandle, INFINITE);
        GetExitCodeThread(threadhandle, ExitCode);
        case ExitCode of
          IDYES: Writeln('Benutzer hat "Ja" angeklickt.');
          IDNO: Writeln('Benutzer hat "Nein" angeklickt.');
        end;
        CloseHandle(threadhandle);
      until FindNext(DateienLst) <> 0;
    finally
      SysUtils.FindClose(DateienLst);
    end;
end;