[NumLock]

-в OnCreate вызвать SetAlignmentJustify:

Код:

procedure TForm1.FormCreate(Sender: TObject);
begin
  SetAlignmentJustify;
end;

-код вставки изменить (лучше наследника TRichEdit создать) на:

Код:

  SendMessage(RichEdit1.Handle, EM_SETSEL, -1, 0);
  SetAlignmentJustify;
  SendMessage(RichEdit1.Handle, WM_PASTE, 0, 0);

-ну и сама SetAlignmentJustify:

Код:

procedure TForm1.SetAlignmentJustify;
const
  EM_SETTYPOGRAPHYOPTIONS = WM_USER+202;
  TO_ADVANCEDTYPOGRAPHY = 1;
var
  pf: PARAFORMAT2;
begin
  SendMessage(RichEdit1.Handle, EM_SETTYPOGRAPHYOPTIONS, TO_ADVANCEDTYPOGRAPHY, TO_ADVANCEDTYPOGRAPHY);
  ZeroMemory(@pf, SizeOf(PARAFORMAT2));
  pf.cbSize:=SizeOf(PARAFORMAT2);
  pf.dwMask:=PFM_ALIGNMENT;
  pf.wAlignment:=PFA_JUSTIFY;
  SendMessage(RichEdit1.Handle, EM_SETPARAFORMAT, 0, LPARAM(@pf));
end;

-вставку с заменой доработать самому