Washington also is expected to select the contracts of pitchers Mike Hinckley, Shairon Martis and Levale Speigner from Columbus.
Bernadina, 24, will join the Nationals for a second stint in the big leagues this season. He entered play on Sunday leading all of the organization's full-season Minor Leaguers in batting average (.334), hits (151) and runs (78).
Casto, 26, has had a pair of stints with the Nationals this season and was a steady bat off manager Manny Acta's bench, going 5-for-18 (.278) in a pinch-hitting role. Casto's six pinch-hits are tied for second on the team.
The 25-year-old Gonzalez will return from a stint on the disabled list. He was placed on the DL on Aug. 14 (retroactive to Aug. 5) with a left hamstring strain. Acquired from the Yankees on July 31, Gonzalez hit .462 (6-for-13) in four games with the Nationals in early August.
Hinckley, 25, will join Washington after posting a 3.16 ERA in 20 relief appearances with Columbus. Hinckley, who began the season with Double-A Harrisburg, was bestowed the honor of throwing the first pitch in the history of the Nationals in the club's inaugural Spring Training contest on March 2, 2005, against the Mets.
Martis, 21, combined to go 5-6 with a 3.64 ERA in 21 starts this season for Columbus and Harrisburg. The ERA ranks fourth among Nationals Minor League pitchers who have thrown at least 100 innings in 2008.
The 27-year-old Speigner earned a September promotion by going a combined 4-0 with a 1.82 ERA in 35 games for Columbus and Harrisburg.
Washington's second selection in the 2006 Rule 5 Draft, Speigner combined on a .208 batting average against and a 3.1 strikeout-to-walk ratio for the Nationals' top two affiliates this season. Speigner returns to the Nationals for a second straight season. Last year, he went 2-3 with an 8.78 ERA for Washington.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.