[NumLock]

создать обработчик события AfterInsert у ClientDataSet:

Код:

begin
  ClientDataSet1.FieldByName('id').AsInteger:=PageControl1.ActivePageIndex;
  {
  case PageControl1.ActivePageIndex of
    0: ClientDataSet1.FieldByName('id').AsInteger:=999;
    1: ClientDataSet1.FieldByName('id').AsInteger:=888;
    2: ClientDataSet1.FieldByName('id').AsInteger:=777;
  end;
  }
end;