First, powering your laptop over USB itself is likely to be impossible. Most modern laptops that I've seen take between 50 and 100 watts to charge, but USB itself doesn't typically allow more than 10W per port. Additionally, your laptop's USB circuitry likely won't accept power anyway.
If you really want to physically redirect one of your USB ports to replace the power plug, I have a much better idea for you. Just replace the broken power plug with a new one that works! It's going to be much safer than trying to retrofit a USB port, and replacement power sockets are available for most laptops. All you need is a soldering iron, a steady hand, and a bunch of screw drivers (which are all the same things you would need if you wanted to do this to your USB port too).