[PhoeniX]

Как-то так?

Код:

  InStr := 'Джентельмены удачи';
  OutStr := '';
  for i := 1 to Length(InStr) do
    if InStr[i] = ' ' then OutStr := OutStr + '+'
                      else OutStr := OutStr + '%' + IntToHex(ord(InStr[i]), 2);