function NumStringToBCD(const inStr: string): string;
 function Pack(ch1, ch2: Char): Char;
 begin
  Assert((ch1 >= '0') and (ch1 <= '9'));
  Assert((ch2 >= '0') and (ch2 <= '9'));
  {Ord('0') is $30, so we can just use the low nybble of the character
  as value.}
  Result := Chr((Ord(ch1) and $F) or ((Ord(ch2) and $F) shl 4))
 end;
var
 i: Integer;
begin
 if Odd(Length(inStr)) then
  Result := NumStringToBCD('0' + instr)
 else begin
  SetLength(Result, Length(inStr) div 2);
  for i := 1 to Length(Result) do
  Result[i] := Pack(inStr[2 * i - 1], inStr[2 * i]);
 end;
end;
procedure TForm1.Button1Click(Sender: TObject);
var
 S1, S2: string;
begin
 S1 := '15151515151515151515';
 S2 := NumStringToBCD(S1);
 memo1.lines.add('S1: ' + S1);
 memo1.lines.add('Length(S2): ' + IntToStr(Length(S2)));
 memo1.lines.add('S2 unpacked again: ' + BCDToNumString(S2));
end;