If the wire for the switch and the bell that is in the existing conduit is carrying 110V, the answer is NO. You can not mix High & Low Voltage in the same conduit, so you will need to run an additional conduit to connect to the second set of terminals.
Is this for your residence or for a business? I question this as you are "playing with fire", in more ways than one. For one, the wireless contact you reference is a water sensor. Meaning it is designed to detect actual water, as in water touching the wireless contact. These are often placed under sinks or behind toilets to be on the look out for a broken pipe, water leak, or flooded home. Not for hooking a water flow device to it.
If this question is regarding a business, there is a whole other set of hoops you must jump through to ensure you are following what ever Code your local AHJ (Authority Having Jurisdiction) has adopted.
If this is for your home, you need to ensure the wireless sensor you purchase is capable of monitoring a Normal Open Circuit, WITH Supervision. Many wireless sensors do not meet these guidelines.
Sorry for the long drawn out answer, but when dealing with fire alarms, you are dealing with peoples lives. Please keep that in mind as you move forward with your project.