I brew all grain so I don't really know what it would take doing it another method. If you are making a 5 gallon batch, 1 cup is probably about right for some lemon flavor. You might want to rack to a few secondary vessels (if you have some) and experiment using various amounts of lemon flavoring. That way you won't ruin a whole batch if it doesn't turn out right!
You will have to let us know how it turns out!