I would recommend to have the emergency swiches on AS-I bus. Each EM-swich would then have a unique adress. The rest is up to you and how you program the PLC and HMI-system. I have done this many times. if you are using simatic S7 series i can give you some more tips about programming this system
I will recommend to get a hold of a small Siemens S7 PLC Packet for beginners and try some of the programming turtorials which follows with the PLC packet. There are some really great examples and guides to basic programming knowledge.
if you want I can send you som datasheets on this packet.