I added a small pocket to the bottom of a 3300 bag. I didn't need it to be big enough for a water bottle, just head lamp, beanie, gloves etc. I didn't spend a ton of time to make it look good but it works perfect.
I made the pocket out of some scrap neoprene waders that I had. I didn't open up any of the seams on the 3300 bag just sewed it directly on to the side. While I was fitting it I just made sure it wasn't so deep (tall) that I wouldn't be able to reach it while wearing the pack. I only put it on one side because I love the pack and wanted to keep it light but just wasn't happy without a place to dump my headlight, snacks, beanie, etc.