Can I Add a Plug to Lights Designed to be Wired In?
No. They're 12VDC, which means if you plug them into your wall they'll go PFFFZZZZZT and make a little puff of smoke. Or worse. 

You'll need a power supply (a wall brick) that provides 12 volts and enough current. You can add mating plugs to the lights or just crimp them together.