The easiest way would be to make a separate piece and sew it inside. You could cast on a few less than the original number, then make it as tall as you want and sew it inside. I would not put the cable on the pocket(s). Just keep it plain stockinette so it doesn't take up so much room.
What you're going to have to be careful of is that the case will still close with pockets inside. It was wasn't intended for that so you may have to alter it. It's going to be a very thick wallet.
Here's one that is meant to have pockets -