For two decades, Igor Tulchinsky and his team of quantitative traders have helped fatten the returns of New York hedge fund Millennium Management LLC. Now, the roughly $28.5 billion firm and Mr. Tulchinsky are forming a joint venture that will allow the Belarussian trader to accept outside money and use a wider range of trading strategies, said people familiar with the matter, showing the value of star traders and the lengths firms are going to in order to keep them happy...