RM-MN gives good advice to avoid any oils that will be a nemesis to brewers. I was hesitant to use an "extract flavoring" knowing it may be likely to throw an artificial flavor into the beer.
I made a brown ale calling for hazelnut to be added at bottling or kegging. Actually, this was the first "extract" I felt was acceptable. Fruit extracts are generally detectable, but this hazelnut really surprised me. Take a look at the hazelnut extract from MoreBeer. There are other similar products I'm sure, just only used the one I mentioned.
Go easy and add small amounts a bit at a time til you strike the right note for your tastes. I added the full amount my recipe called for, but wished I had added less for a more subtle flavor.