Don't just add 12 inches the color and/or finish will be impossible to match exactly. Replace the entire chain and wire at the same time. Lamps from many of the big name lighting houses can be ordered with longer chains.

Lengthening chain and wire and installing a light fixture is easy if you have the right tools and instructions. Check out the how-to-do guides at home depot if you haven't done this before.

I assembled and installed a heavy crystal chandelier so it can be done at home.