Looks like there might be a place for it on the board. There is a three hole solder point labeled VBB.
To run a Smoothieboard without the USB connected requires an external 5v power supply. The other option is to add one of these switching 5v regulators to your board which will then pull logic power from your main 12v/24v power source: shop.uberclock.com/products/switching-regulator-dc-to-dc-5v
The one they sell is only 500ma, which would be enough. I have some that are 1 amp for $5, but they mount 90 degree to the board so we would half to see if they fit.
Some things are meant to be closed. Your mind isn't one of them.