You will have to see how the circuit is wired. If the switch is "key" which means the original electrician only sent a switch leg down to the switch from the light you cannot use it. If you have a neutral than you can.
The easiest way to check is to see how many wires come from the switch box. If there is only one, I highly doubt you have a neutral which means you need to get your power somewhere else. If there are multiple wires coming from your switch box then you might have a neutral and could tie into that box.
Master Electrician, Air Conditioning & Heating Repair Company Owner. www.TECertifiedElectricians.com