Yes you can use the existing thermistor, providing you can get it off without any damage to it. They aren't that expensive.
You're going to want one of these:
http://www.ebay.com/itm/30-Amp-Relay-Bo … 3cea59a775
Also you're going to need a:
http://www.ebay.com/itm/12V-DC-30A-360W … 2eda42a107
The reason being is if you run the power to heat the bed through the board, you run the chance of burning out the existing power supply or the controller board on your printer, maybe even start a fire.
SD3/SD4 Mods completed: glass bed, changed zrod to 5mm, slop nut, aluminum arm-originally wood, plexi-glass case, Z Wobble Preventer, Lawsy Carriages, X\Y motor fans, control board fans, extruder cartridge heater, MK5, Esd Lite
Mods working on:Direct Y Drive
Remember.......All you touch and all you see is all your life will ever be....Pink Floyd...Dark Side of the Moon.