You can put a hydration bladder into any pack, new or old. Just because it wasn't a design feature for when the pack was made, doesn't mean you can use one. Just open the top and put it in. On those old externals, just run the tube out the top flap. However, Lori's advice is really the best place for one. If I used one, I would put it on the outside of the pack, on the very top. That way, when it leaks, it doesn't soak the internals of my pack, I can refill it easily, and it is in a good spot for weight.
Another good spot to put it is behind your shoulder straps between your back and the pack bag, outside of the pack.
I don't like hydration bladders. I carry water bottles on my hip belt. Sure, I have to pull it out, but it doesn't take much effort, and I don't have to put the pack down and dig it out. Also the weight is directly on the hip belt. And, if it leaks, no big deal. But, I have never had a single cheap water bottle leak on me....which I can't say for the three camelbacks I bought.