Hops are added for three primary reasons: bitterness, flavor, and aroma. For bitterness, you boil them for roughly 60 minutes. For flavor, they are added within the last 20 minutes of the boil. And for aroma, they are added at the very end of the boil, or at some point after. Most people that make IPAs will dry-hop the beer, where they add hops to the fermenter, usually after the primary fermentation is done or nearly done. In regards to your question, the hopped LME will have already had hops additions done for all three characteristics. But, if you would like to add hops just for aroma, you could add pellets using the dry hop method. Its pretty easy, there are different approaches to this, I'd just do a google search or browse around on the forums here for some basic techniques.