The University of Pennsylvania has joined a lawsuit accusing the president of the Memorial Sloan-Kettering Cancer Center of walking off with the fruits of university research to start his own company. The Sloan-Kettering president, Dr. Craig B. Thompson, was sued in December by his former workplace, a cancer research institute located at the University of Pennsylvania, which said Dr. Thompson had concealed his involvement with the company. But on Wednesday, Penn filed its own complaint against Dr. Thompson in Federal District Court in Manhattan.