too lazy to set up a private match to test this, but a while back I hacked an enemy level 3 rocket turret on Giii. A couple seconds before round ended I hacked the level 3 turret again, and although I know it will say 3.X, it looked like the turret was firing at 3.3 can you re-hack, hacked enemy turrets?
You can rehack enemy turrets and it's the equivalent of a .1 hack. I tested it awhile back on a shaveice.
you will want to rehack enemy turrets because the duration works the same as on your own, but instead of losing the .X the turret returns to the enemy team
On a side note, an enemy Support hacked one of my Rockets a while back and it started killing my teammates. MY MOST EPIC ROUND THE HORN!
Guide to most epic round the horn. Play a match against a team of 4 Supports. Run back to your base when pitgirl says ur ball down. See blue Rocket that doesn't belong there. Run into spawn and taunt the kills your ex-rocket is getting you. Proceed to be generally epic in every way.