Kimball was already on Syracuse's roster, but outrighting him takes him off the 40-man roster, opening up a spot.
The Nats now have 39 players on the 40-man roster. That does not include reliever Christian Garcia, who is on the 60-day disabled list and is currently on a rehab assignment with Syracuse.
Kimball made his big league debut with the Nats in 2011, posting a 1.93 ERA in 12 appearances. The hard-throwing right-hander struck out 11 and walked 11 in 14 innings, but then underwent season-ending rotator cuff surgery that set him on a long rehabilitation process.
He showed up healthy to spring training this year but has appeared in just four minor league games all season.