Yes, though you should boil it in water first. You can also add some sugar, though that won't taste as good.
If you leave it you will have basically a Mild. Though with a lager taste. Around 3-3.5 a/v. Will still be enourmously better than "light beer" megaswill!
I'm guessing the recipe was for 5 gallons and you topped up a 6.5 gallon primary which would bring the OG down form 1.047 to 1.037.
To bring the OG back up you'll need about 30oz (weight) of LME.
If you don't have room in the primary, then siphon a quart or so into a pot and boil with the LME (or DME) for 20minutes, cool, then add back to the primary.