Adding a switch to interrupt power to the magnet should not be any problem. As a matter of fact, some manufactures install these buttons on the above-the-door door holders. The worst that can happen by installing the power interrupting switch is that if the switch fails, the door closes.
Basically, if the switch fails, it will fail safe.
This means that if the switch fails and continues to apply fire alarm system supplied power to the door magnet, the fire alarm system will still be able to close the door in case of fire, it will just be difficult to close it manually.
On the other hand, if the switch fails and turns off the fire alarm supplied power to the door magnet, the door will close whether or not the fire alarm system has detected fire, someone just has to hold it open for the door to stay open.