I think these days you would be far better off using regular beans and buying a hazelnut syrup to add to the drink.
Several of the sponsors sell them. I know that Coffee Hit have them on their web page.
The problem with infusing the coffee itself with a flavour is:
If you are buying it pre-ground, it is stale.
If you are buying whole beans, you need to put them through a grinder and that will taint the grinder.
Hope that helps.