Over its three-decade existence, the Japanese conglomerate SoftBank and its founder, Masayoshi Son, have been known for ambitious and sometimes head-scratching moves. Now, SoftBank is nearing its most unusual move yet: It is buying Fortress Investment Group, an American private equity giant that oversees around $70 billion in assets. That business is a radical departure from the technology and telecommunications holdings for which the Japanese company is known. But the acquisition is intended...