If it's metal open one of the knockouts at the top of the lower box and slide a plastic Romex connector (with the leading end pointing up) 5 feet down a coil of wire. Attach the cut end of the Romex to a short piece of snake and thread the snake and Romex from the inside of the lower box to the wall opening above which you have properly located and neatly cut to accept your new box. Pull the snake until the connector stops at the top of the lower box, then push the conector up until it snaps in place. If the box is not metal then the best way to enter the lower box is from the outside [(send the snake up through one of the slotted one way openings, attach the wire from the new hole and pull 1 foot of wire down into the existing box). Don't use a connector]

Arlington makes some nice boxes for this application.

Here is a link that might be useful: wall mount TV box

Here's one supplier.

Here is a link that might be useful: monoprice