How to use an normally closed hall effect switch as normally open?
Using a normally closed Hall Effect switch in an application that requires a normally open action is easy to achieve. You will just need a suitable circuit that "inverts" the behavior to give the output you need. That will take a few low cost additional components and may require the availability of a voltage rail from which the "invert" circuit can be powered. 

Specific details are not easy to provide because you have told us little about the nature of the Hall sensor and your switching requirements. But that said please consider this simple concept to get the idea of an "inverter" circuit across. (Note I am not suggesting that this would be suitable to your application - just an example).

Envision that your normally closed Hall Effect sensor is wired up such that it is keeping an NPN transistor in the OFF state. When the sensor changes state it goes open and allows the NPN transistor to go ON. This NPN transistor is in turn connected to the coil of a small relay that gets energized when the NPN transistor goes ON. The contacts of the relay provide the switching action that you require. If it has FORM C contacts (a.k.a double throw) then you have your choice of inverting or non-inverting switching action relative to the Hall Sensor.