How can I add an internal pocket to pants that don't have one?
You could sew a passepoil pocket (I don't know if there is a more common way to call it in English), and maybe use a zipper to close it. If there is a seam at the side, you might want to use that as the location for the pocket.
Anyways, for instructions (in Dutch, but with a lot of pictures) you can check http://fromfabrictofashion.nl/diy-de-paspelzak/ . If anybody knows an English site, please feel free to edit!
Basically it's about sewing a rectangle to the right side of the fabric, then cut through both layers of fabric, and pull the rectangle through the hole. You can attach the pocket to that rectangle. Or, if you make the rectangle large enough, that could already be the pocket.