What you do is put in your LED undercabs in and the driver is inline before the light, than the driver is hooked up to the undercabs. You don't need to add anything other than the original switch that will switch on and off the undercabs.
Sometimes the drivers or low voltage transformers (depending on type of lights used) can be placed inside the cabinets. You can also get LED undercabs that have the drivers built in, so no drivers need to be wired in. The new Philips ones are pretty good. Be sure to match the overhead lighting color temperature with your undercabs. You don't want a 4500k overhead light with 3000k undercabs, it looks bad.

Mike