I did with a casonade caramel. It was really good.
Caramel isn't fat but indeed not easy to incorporate. I will advice you to make your caramel and the dissolve it into some apple juice or some of your cider (but you will lose alcool doing so) and thne throw it into you fermenteur when it is still verry hot. Then shake stir and bottle. It will be still obviously.
For the quantity, I always think in density, I like my cider around 1010 and 1005, depending on how sweep I want it. In your case, I will definitly go to the sweet side so 1010, maybe 1015 if you really want it sweet.
So if you cider is bone dry ±1000, you can add between 20 and 30g of sugar for each liter and it will give you and FG of 1008 or 1012.
The way you will make you caramel (water quantity, colour,...) won't change the amount of sugar you actually add into you cider. I don't think that caramel give a different sweetness than the sugar you use to do it. Anyone can confirm?