Can a porch ceiling fan be installed using the wiring controlled by a 911 emergency light switch?

Depends on how it is wired. If power could be pulled before the 911 switch it is possible but we have no idea how your wiring is installed.

The immediate response is no! My chief concern is that the alarm system may be a low voltage system (24 Volts). If it is a 120 Vac circuit, you still cannot pick up a complete circuit at a switch.

You cannot pick up power at a switch unless a true Neutral (White) is present. Switches interrupt the Hot side of the supply conductor only. If a white and black are connected to a switch then it is a switch loop. White connects to the supply side black and should be marked with tape or marker (any color other than gray or white) to show that it is now a hot conductor.

Recent code changes now require that Neutral be brought to a switch outlet via a three conductor cable. Neutral is capped off at the switch outlet if it to be unused. Black connects to the supply black conductor and red returns the power to the circuit.

Rather than use a 9-1-1 system, does your porch have a light that is controlled by a wall switch?

If you do, then you can remove one of the lights on the porch. Remove the lighting outlet box and replace it with a fan bracket listed to hold the weight of the fan.

You can wire it this way, connect the black from the fan to the black supply wire. Also connect the white supply wire to the black supply wire and capped the 3 wires together. Now, connect the black wire (return from switch) to the blue (fan light) if the fan has a light package.

Now, the fan will be hot all the time and can be controlled by a pull chain or remote. The light will work off the switch.

Connect the white from the fan to the white supply conductor.