Sprint Nextel Corp. (NYSE: S) and Japan's SoftBank Corp. are officially seeking federal approval of their pending merger, the Kansas City Business Journal reported. The newspaper, citing documents filed late Thursday with the Federal Communications Commission, said that SoftBank intends to buy 70 percent of Overland Park, Kan.-based Sprint in the deal valued at $20.1 billion. Sprint has extensive operations in the Dallas-Fort Worth and is the nation's third largest wireless carrier behind Verizon…