What you are asking is basically impossible, or at least extremely difficult. I also think that you actually want something different.
A device that has an input anywhere in the 6 - 30 V and outputs 500 W at 240 V would have to draw over 80 A at 6 V, falling to 16 A at 30 V.
When you say that the Nokia gadget outputs 6 - 30 V, that is a very wide range for a DC supply. I guess that the gadget can switch a voltage in that range, so if you provide it with say 12 V, you have 8 outputs that are either 0 V or 12 V. I would also be surprised if the Nokia gadget could switch or supply more than 2 A or so.
Then we come to power. Where is the power for this device coming from? 8 outputs at 500 W is 4000 W. If that is coming from a low voltage source, say 24 V lorry batteries, it's over 150 A which is a serious current and well outside the scope of an amateur. If the power is coming from a mains supply, it's plain stupid to transform it down to a low voltage, switch it, and transform it back to 220 V.
If you want to control 8 devices, each up to 500 W at 220 V, from a device that can switch a low voltage, you need a low voltage power supply, and 8 solid state relays.