The first hop addition are your bittering hops. The long boil is for alpha acid isomerization. Simply put the longer the boil, the more bittering you get. Dry hopping will not add bittering. It is more for hop nose, that wonderfu smell and some flavor. There is no reason not to dry hop your beer so why not? It may not turn out as bitter as it is intended to be but it probably will still turn out to be great beer.