The trouble is, USB operates at 5V. Even if you tried to send that 5V through Cat5/6, it would not get very far because the cable itself has some resistance and it will reduce the voltage down to a point where it won't be usable very quickly.
Honestly, I wouldn't bother with USB cameras for a security system, mainly because of this limitation. USB can only send the signal up to 5m.
I would suggest buying IP cameras instead and then power them via POE. You just need a POE switch to send the power, like this one: http://www.panoptictechnology.com/pse-8-smart-switch/
And then you can either buy cameras that support POE, or buy POE adapters like these to power non-POE cameras: http://www.panoptictechnology.com/network-smart-adapters/
It makes the install nice and clean since you only have to pull one cable out to each camera.