Some people use amylase or "Beano." The enzymes break the longer sugar molecules into simpler sugars that can be consumed by ale yeast. This will make the finished beer bone dry, and you cannot know how low the finished gravity will go. Amylase (alpha and beta) are the same enzymes present in the mashing process, so their action is probably limited to some degree. Beano can break almost any sugar, acts slowly, and may result in overcarbonation/ gushers/ bottle bombs if it keeps on acting.
Why do you want to have a lower FG? Is the beer too sweet, or do you want a little more alcohol? I would really recommend letting it as is, and shooting for something different on the next batch.