Bags that are long enough and sturdy enough to take a hip belt probably already have them. The belt needs to couple well to the frame/load if it's going to take any weight so you need more than just sewing skills. This doesn't mean it's impossible, just hard, and the best source of materials might be a hip belt off an old rucksack. The only reason I could see to want to add a hip belt is for a challenge, unless the pack is very special.
A stabilising waist belt doesn't take any significant weight, but would be much easier to add. This can be as simple as two pieces of webbing with an adjustable buckle. Waist belts are often attached to the same points as the bottom of the shoulder straps, so stitching or knotting a belt there should work (it would probably ride up if just clipped). You could cut a webbing belt in half, seal the ends, and tie it on, for example, though I'd put a few stitches in to get the angle about right.
As it's mainly storage you're after: backpacks that lack hip belts are often comfortable over a separate belt, either with built-in pouches like those sold for running/cycling/cameras, or with extra pouches threaded on. Such a belt could be hung from the backapck to keep them together, using a couple of lengths of paracord to the bottom of the shoulder straps.