January 26, 2010 Berkshire Hathaway to Join S&P 500, Shares Soar By REUTERS Filed at 6:39 p.m. ET NEW YORK (Reuters) - Standard & Poor's said it will add Warren Buffett's Berkshire Hathaway Inc <BRKa.N> <BRKb.N> to its flagship S&P 500 stock index <.SPX>, as the company prepares to acquire railroad operator Burlington Northern Santa Fe Corp <BNI.N>. Berkshire's Class B s..