you can make your own extract with 4 parts grain alcohol to 6 parts distilled water (making around 40% ABV). blend up the nuts and put them in a canning jar with the liquid for a few weeks. if there is a visible oil slick on the top, put it in the freezer to solidify the oil and then skim it off. then just pass it thru a coffee filter and add it to the keg.
i ususally do a taste test before adding, because the stregnth of the mixture is so variable. pour a few 1oz cups of the beer, and use a medicine dropper to put varying amounts in each cup until you find the stregnth you like, then just do the math to scale it up for the entire keg.