I've done it with two relays mimicking the switch. One is a standard SPNC (connecting the indicator relay to the stalk feed) the other is a Double Contact/Double Output relay (like this one) attached to the left/right circuits.
The general working is that in normal conditions, the SPNC connects the indicator relay output to the stalk. When the hazard button is pressed, both relays are activated, disconnecting the indicator relay and connecting both left and right indicator circuits to the hazard relay.
This can be dropped into a standard loom without much cutting/joining. All you have to do is source an ignition switch +ve connection for the switch/relay coils.