It would be the same principle as backsweetening a cider or adding extract flavor to beer bottles.
Just make some strong coffee "concentrate" then add it to the keg and give it several light swirls with your mash paddle/long spoon.
As far as how much to add......start small, give it a taste, then add to taste.
Primary #1 - Midnight Ryeder (Midnight Wheat and Rye)
Primary #2 - Florida Weiss
Primary #3 - Kane-DOH APA (Honey Citra APA)
Secondary #1 - Downtown Flanders Brown (brewed August 2012)
Keg #1 - Raspberry Florida Weiss
Keg #2 - Cinnamon Raisin Cider
Keg #3 - NONE!
Bottled - NONE!