Yes, they do list a drink tube kit on the Cascade Desgns website , but I agree with LoneStranger that two or three smaller bags are better than one large. Unless you're always in the desert, most of the time you won't need to carry one or at the most two liters at a time. Even if it's dry the whole day, four liters (a little over a gallon) should be enough unless you're going to have a dry camp as well.

In an area with frequent water sources, you can save a lot of weight by getting water several times per day instead of trying to carry a whole day's water at once. At 2 pounds per liter, 6 liters are a lot of weight! It also helps to get a good drink at the water source, better known as "cameling up." Up high in the Rockies or Cascades, I often don't carry more than a half-liter at a time.

Another problem with one large bag is that if you drop it when full, it's far more apt to burst than a smaller bag (been there, done that, brand new Platy 3-liter bag, easy to drop when full and slippery). I'd suggest 2 or, at the most, 3 two-liter bags instead. The ones you don't need can be rolled up to take basically no room in your pack.