In 120V land, Insteon will do that--you can use normal Insteon controls in the four locations that you've already wired, and install wireless switches in the other two spots. It should be pretty much seamless.
Also, if you can get wires from any
legal circuit to the to two missing spots, standard wired Insteon controls could be used there to control your "6-way" circuit.
I don't think they have a 230V version of Insteon, though.