I haven't personally done it with nuts, but I've made my own extracts for beer using coffee and chocolate in the past.
Get hazelnuts, grind them up and soak them in high proof vodka or grain alcohol for 2 weeks. You could also use bourbon too, I personally prefer that for additional flavor.
You'll only need 2oz or less of extract for 5 gallons. You'll taste it for sure. Id say use about a handful of hazelnuts and half a cup of liquor. Shake periodically during the 2 weeks. Once done, run your mixture through a coffee filter once or twice and your done! The alcohol will extract the color and flavor of the nuts.
It'd be interesting to do this side by side with toasted and raw hazelnuts to see which flavors you prefer.