Assuming the cans and half-gallon are all sealed, you should be able to just spray them down with sanitizer, open and pour into your keg (gentley to avoid oxygen exposure). Then seal the keg back up and flip it over a few times to mix thoroughly. One note, if you've already carbed the cider it may foam up when you pour the concentrate in, so be aware if your liquid level is close to the top of the keg.
I'd advise adding in stages and tasting in between so you don't oversweeten! I'm sure you're aware but once you backsweeten you can't pull the keg out of the fridge, as it will start fermenting the added sugars if you let it get back to room temp.
I'm doing my first cider now, let it ferment down to 1.008 over 12 days, and just kegged it last week. Before sweetening it tasted almost like vino verde, pleasant but not the cider taste I was looking for. I backsweetened with 2 large (16 oz) cans of generic apple juice concentrate. That seems like a pretty good sweetness level to me, though you might want more if you let it ferment further down or if you like it sweeter. I can't comment on the value of long aging though many recommend it. Mine tastes good already uncarbed, though there is a very slight sulpher smell at first (disipates quickly) that I think I've read may mellow with age. I also tossed 3 teaspoons of malic acid in mine to give it more of a "sour apple" flavor -- I gather that pressed cider has more of that than the Motts juice I used, but have also read that more apple character comes back with months of aging. Not planning on letting this first batch hog a slot in the kegerator for six months though, so I won't be finding out this time around!
