A former official of the Federal National Mortgage Assn. was charged with embezzling about $770,000 from the agency over about a two-year period and diverting the money to his own use. The charges, detailed in a three-count complaint released by the U.S. attorney's office in Los Angeles, named Robert Masato Tanabe, 38, of La Canada. Tanabe was the mortgage association's manager of property disposition from May, 1982, through July, 1984. In that position, according to Assistant U.S. Atty. Gary Feess, Tanabe purportedly diverted more than 300 checks from foreclosure sales and deposited them in an account he had opened at the Montrose branch of First Interstate Bank. If convicted, Tanabe faces a maximum five-year sentence and $5,000 fine on each of the three counts.