eMate power supply

Devices that are designed to run at either voltage usually have a selector switch on it (like a computer's power supply) or have a modification to the internal wiring scheme (like an electric motor). Physical conversion of the plug type is only half of the equation. An AC transformer is needed to step down the voltage as well, or the device will be damaged when connected to the higher voltage.

"... the line voltage must be 100-220 volt AC."

Devices labeled like that will work at 100 through 120 Volts AC. (The Voltage used across the USA.)
They will also work at 220 Volts AC. They will also work at any steady AC Voltage between the two extremes.

You do not need a step-down transformer or power supply. All you need is a plug adapter from parallel blade (USA standard) to whatever plugs you usually use (you showed a picture of a plug adapter with two round pins.)

If there is a switch, you definitely need to set it for 220 Volts AC before powering up. If there is no switch, you just plug it in and go, the unit will adjust itself automatically. Many PowerBook adapters work this way -- in 220 Volt countries, the same PowerBook power adapter is issued with a different line cord that ends in a commonly-used plug for that country. No switches or other changes are expected -- the unit adjusts automatically.

The stock eMate charger is the common "Newton 9W Power Adapter". Input: 100-240 volt 0.5A AC 50-60Hz with output at: 7.5 volt 1.2 amp DC

I have several adapters for later Newtons and an eMate and they are all the same.