Mash at a couple of degrees higher than the recommended or keg the beer before the fermentation has completely finished and all the sugars have not turned to alcohol.
As for sweetening a beer that is already in the keg - I don't know.
edit: Opps I see that you are a K&K so forget the first suggestion.
hi,
my 5 cents worth on this one, is to release the pressure and add some dissolved lactose, or an artificial (coffee )sweetener. if you are consuming in a short time then a brewmaster told me to add some honey.