My recent experience went like this with a 23 litre Blackrock Cider can made from New Zealand grown apples:
Bugger the SG, the ferment (which had been a bit slow) was clearly nearly at an end. Tasted out of the fermentor: not bad, some sweetness, the wife does not want it any drier.
Decide to wait a day or two until I see some clearing at least part way down the fermentor. The intention is to go straight into a keg & refridgerate at 2 to 4 DegC.
We do that but the wife says half way through the siphoning: "I'd like it a bit sweeter".
Fair enough, I add 1.5 litres of supermarket bought (no preservative) apple juice to the keg & top to 19 litres.
After forced carbonation & only about 10 days aging, we sample:
It's OK, no weird or off flavours, a wee bit hazy still, a clean finish, some apple flavours, but its watery (my wife's description with which I agree), the sweetness is about dead right, best described as on the dry side of medium.
So backsweetening with apple juice works & is straightforward. I don't know if the juice was pasteurised but suggest there was not sufficient used to spoil any flavour; it's sometimes said that pasteurised juice results in an inferior taste.
I followed the kit recipe which called for 1 Kg of cane sugar - except I used 1/3 KG dextrose & 2/3 Kg of dark brown sugar instead.
Obviously the low temp at which I hold my kegs has stopped any further fermentation as you would expect.
What I'd really like to know is how next time do I get more of a taste kick, watery is definitely not good enough