you cant just add freon to a system without pulling a vaccum first.
this is true for all cars.
anyway to answer your question. you are NEVER supposed to forcefully turn the compressor on by bypassing any sensors. when you fill the car with gas you have to have the a/c button on inside of the car. the can of freon being at a higher pressure then your a/c system or in your case a system at near atmospheric pressure, will flow from a high pressure to low pressure. kind of like filling your tire with air. once the pressure inside the system becomes high enough the pressure sensor in the system gives an ok to activate the clutch on the compressor thus turning your compressor.