Yes, the Apple Extended Keyboard (the original as well as the Extended
Keyboard II) can be used with the Macintosh Portable, as can the Apple
Standard Keyboard and other ADB devices that follow our specifications.
The differences will be in key layout, battery drain, and waking up the
Macintosh Portable.
The Apple Standard Keyboard draws a maximum of 100mA, and the Apple
Extended Keyboard draws a maximum of 85mA. The Macintosh Portable
keyboard and keypad (or trackball) together draw 75mA. From these
numbers, you can see that adding the Extended Keyboard more than doubles
the ADB power requirements. This decreases battery life somewhat. Since
you will be using the Extended Keyboard with the charger plugged in, this
shouldn't affect you.
Another point, though minor, is that the only way to manually wake up the
Macintosh Portable is by pressing a key on the built-in keyboard.
External ADB keyboards cannot provide this functionality. You will have
to power up the Macintosh Portable by pressing a key on the built-in
keyboard instead of the externally attached ADB keyboard.