All that the indicator switch does is to complete a circuit, the result is that the chosen two lights flash together.
You could install a switch (no relay required) that completes the circuit to both left and right lights.
Bear in mind the flasher unit will need to be strong enough to handle 4 bulbs, you do get heavy duty units. You will be drawing more current than when only two bulbs light up, however your bike should have watts extra in reserve.
It should be easy to find the correct cables from the handle bar indicator switch easily.
Make sure that all connections are waterproof.
Remember that should you bike be under guarentee it is probually better not to tinker just yet.
To those who say it can't be done - stand aside for those who are already doing it.