Extract actually works very well here. Hold back about a gallon of the water you would have topped off with, and ferment as usual. After a week or so, add your berries and that gallon of water to a large pot and bring to at least 160F for at least five minutes. When cooled to under 100F, add the berries and water to your beer and let ferment for another couple weeks. You'll have a ton of trub and some beer. I'm not familiar with huckleberries, so I can't tell you if 7lbs is a good amount, but I'm guessing that's a little on the high side.