Well, the boarduino already has that connection.
With mine, I do just that... I have one of those nice breadboard plugin supplies and attach it and the boarduino 5V pin to the 5V rail on the breadboard.
Later on, you would just feed 5V to the 5V pin and move the USB power jumper. The jumper would let you isolate the USB hardware.
The PWR solder-jumper is a bonus for advanced users, it allows you to run the USB Boarduino at a different voltage. It is by default shipped with a trace connecting the two jumper sides, which means that the AVR is running at 5V from the USB port. If you would like to say, run the AVR at 3.3v, you can cut the jumper with a sharp xacto/craft blade (check with your multimeter that it is no longer shorted) and then on the breadboard, connect the *VCC* pin to the 3v pin. This will run the board off of the FTDI chip's internal 3.3v regulator, which we should warn you can't supply more than maybe 50mA. Also, be aware that running a 16MHz AVR at 3.3v is considered 'overclocking'. We've never had a problem with it for hobbyist use but just be aware that this is outside the guaranteed performance of the microcontroller