It's so easy a caveman can do it.
If I had to do it, I would wire parallel to the temperature sensor. All you have to do is take the wire that connects to the temp sensor to gound. So, get you a switch and some wire. You will have to check the switch capacity to make sure it can handle the load. If you are going with a microswitch you may need to do a relay then. But just a "normal" switch is easy. Just wire one side to gound and connect the other side to wire that connects to the temperature sensor. Keep the temp sensor it the circuit, so I would keep the orgianl wire there and connect the wire from the switch...
COnfused, it's harder to write about it and explain it, then to actually do it.
Relays are just as easy, just a different type of switch...
But why do you want to do this???