I want to add a light and switch in my hallway from the power in my bathroom light
Depends on if the incoming power is runing to the light or the switch first.
incoming power is to light first
Kill the breaker first!How to Install an Electric Switch
This can also be done with a wireless switch. Heath makes one.
Also depends on if the circuit also contains the bathroom receptcle. If it does, then by code you can not tap this circuit to run devices outside the bathroom.
Is there an outlet in the hallway? If so you could tap off of that.