Just to let you know along with what everyone else has told you, the image data cannot be expanded because it didn't exist in the first place. Digitally, the pixel size is already set. So when enlarging the photo size the pixels are added but the reason it will look poor is because the computer is "making up" the new pixels.
One thing that might help (maybe) is printing out the photo at its current resolution, then scanning the print at a very high resolution, atleast 500 dpi. It's a method i used once and it worked as much as I needed it to.