That's unpossible. You can't magically add more colors to an image that already exists. The color information for an image is already there. The only way to get a higher color bit depth would be to use a camera or program that creates images at that depth from the get-go.
You could maybe TELL the file that the depth is 48 bit. For example, you can save an 8-bit (24 bit) image in photoshop as a 16-bit (48-bit) one, but it wouldn't actually increase the number of colors.