You definitely need to purge the O2 out of the keg. And you're going to need to fill the keg with some pressure to seal it (20+psi). I don't know how much sugar you need. Check out this thread
. I'll quote what seems like sound advice:
Originally Posted by Yooper
The easiest way is to prime as usual, ... then fill the keg.... Headspace isn't an issue at all- you can put the co2 on the keg, and purge the headspace several times to displace any air with co2.
And it will carbonate quite a bit in the keg, after priming, but you can also force carb to correct any lack of carbonation. It might even have more carbonation than you desire. Usually you use 1/2 the amount of priming sugar for a full keg, so I'm not sure how much you'd use for a keg that's only half full. If it's a little overcarbed, that's easy to fix just by releasing the pressure a few times.
So, you can prime with 1/2 as much sugar as you otherwise would have, and force carb if it comes up short. Or you can use the originally called for amount and purge if it's over-carbed. Since you want to carb it with the priming sugar, I'd go with the original amount and purge if necessary.