It is cheap and easy. Have I done it? No, I don't like hydration bladders that much (I have 4).

4 options for the pocket.
1. Go to walmart. Look in the $1.50 fabric section. Buy half a yard of whatever fabric you want. I would recommend something that is not cotton, but it pretty much can be anything. Cut a piece the size you want and stitch three sides with the top not stitched. You can also buy fabric somewhere else for more money. But you won't need more than 1/2 a yard. You won't even need that much. So, if it costs $10 a yard, you only spend $5.
2. Get some straps and sew a few straps inside to hold the bladder against the back.
3. You can also get a closed cell foam pad and cut out enough to cover both sides of the bladder in an envelope. Then sew this to the back of the pack. Now you have an insulated bladder.
4. Don't worry about a pouch. Just cut a small hole in the top of the pack for the hose, and put the bladder inside. It is not like it is going anywhere. You can also make that CCF envelope for insulation and just put it in the pack without sewing it to the pack.