If you can, get your hands on instructions for Elizabeth Zimmerman's After Thought Pockets. I know you can find instructions in _The Sweater Workshop_ and also in one of EZ's books (blanking on the title...). It's sooooo easy to add pockets to any sweater. Let's see if I can break it down for you:

Pick up your finished sweater and decide where you want the opening for the pocket and how wide you want it. Mark the center stitch of that spot. Take a deep breath and cut that stitch. Pick apart that row of stitches on either side, until the pocket is as wide as you want. Put the stitches that are now live on double points--two needles on the top and two on the bottom. Add yarn at one corner and start to knit. Purl the first row (for a turning row) then work in stockinette in the round until the pocket is as deep as you want. Bind off (I do a three-needle so I won't have to sew up the bottom when I'm done). Weave in the ends, including the ones left over from when you made the original opening. Viola! Pocket! A friend of mine then secures the bottom of the pocket to the inside of the sweater so it won't shift around. I just leave it 'cuz I'm lazy.