You can not retrospectively add information the camera did not gather at the time, but as mentioned, there is software that can process your picture files and guess what a pixel in between 2 you have information on would have been, had you taken the picture at a higher resolution. If you follow :-)
JPG format creates smaller files than might otherwise be the case, by leaving out information it believes your eye will not miss. The amount of information discarded is settable, although there is a knock on effect on file size, and on picture quality. But ultimately the control you have is down to what your camera allows you.