Nano Hardware Hack to increase power to servos.

it has already been mentioned - but just use a relay. the arduino switches the relay, and the relay provides the high power. very straightforward and simple.

Why don't you just use a motor shield? Other than that you can use a relay, Diode, or a number of isolation circuits.

Rather than slop answers all over this page, most of which were drawn from deep inside the commenter's bowels, I thought I'd suggest getting a programmable, or at the least, a current limited variable lab bench supply. Very handy when you've accidentally fused a Vcc and ground pin together with the soldering iron purchased from the crackhead on down the way for a bag of cheeseburgers.

I see diodes in your future,and also voltage regulators.

Primo - you will fry your arduino, cutting pins won't do the job. Just get a dc-dc converter for this 7V PSU.
Secundo - your servos won't like the 7V, most of them are rated for 6V max and 6,5V worst case scenario.

The shielding of a USB housing is connected to the PC which is also connected to earth ground. Ground on an arduino, when connected to USB will also be earth ground. If it's critical you can purchase a USB ground isolater. Dave at EEVblog has a few videos to take note of, particularly the how not to blow up your oscilloscope by avoiding ground loops, one in which he directly shorts a power rail with his scope ground lead. Don't connect more than 12v to an arduino Vin. 5v is a pin that provides a regulated voltage to a project. Perhaps if you use another voltage regulator to obtain 7v or supply 7v, you can use that. I've personally used 12v to Vin while the PC is plugged in though, no wuckers as Dave would say. I should note here that my Korad KA3005P DOES NOT have a jumper from earth ground (green terminal) to power ground (black terminal). Try to isolate your power supply from earth ground and you shouldn't have a problem.

Yeah just disconnect the 5v line from the arduino and wire your higher voltage just to the servo power lines. You will still need ground from the servo tied to ground on the arduino. Fyi, not all servos can handle more voltage so check specs first. Also, if you want to be extra cautious you can use an optocoupler between the servo and arduino for insurance, but this isn't necessary.

If you want a schematic of the wiring, Google "wiring high voltage servos" and some rc websites pop up with good images.