The other answers are all correct, you cannot, but they don't address your question of how this works in theory, so I'll take a stab at it.
You can think of voltage sort of like water pressure, and amperage sort of like the velocity of water flowing. Right now in that socket is 250V worth of pressure. Since nothing is connected, no amps are flowing. That 20A is the max amount of amount of flow the socket can support till things in the socket start breaking.
Your microwave is only rated for 100V of pressure, so if you hooked it up to the 250V, it'd be like hooking a garden hose to a fire hydrant: the pressure would be too much and it burst, probably allowing no current through, though it might allowing unrestricted amps until it exceeds 20A worth of flow and the breaker would trip.
The 15A on your mircowave means that it is guaranteed to allow 15A or less to flow under normal voltage and operation. Most devices work sort of like a valve, when off allowing no amps to flow, and under different conditions allow more or less amps to flow. Generally, as long as the amps required by a device is less than the max allowed by your power source, you're fine.
This is all governed by Ohm's law, which if you google you can find far better examples, explanations and analogies than I could come up with.
How does the voltage come into play?
... voltage is the "pressure" that forces current to flow ... increasing the applied voltage will increase the amount of current that will flow in a circuit, possibly beyond the capacity of the appliance