How do I map a key on the A1048 109-key Keyboard to provide an "FN" button?
You will not be able to do this.  The controller in the keyboard sends a keycode to the OS whenever a key is pressed.  That keycode is unique to every key on the keyboard... except for the Fn key.
The Fn key, modifies other keys on the keyboard, as does Shift, Alt, Ctrl, Option, etc... the only difference is that each of the latter modifiers is recognized by the OS.  In other words, the keyboard controller tells the OS that &quot;shift&quot; was pressed, at the same time as the &quot;s&quot; key.  The OS then iterprets that to mean the user wants a capital letter &quot;s&quot; or &quot;S&quot;.
This is not the case with the Fn key.  The fact that it is pressed is captured by the keyboard controller and a separate, unique, keycode is sent when you press the next key.  For instance F1 sends a keycode, lets call that keycode &quot;xyz&quot;   When Fn and F1 are pressed at the same time, a different keycode is sent to the OS.
What you could do is discover what those keycodes are...the ones that are issued when you press Fn + F1-f13.  Then if you wern't using the function keys for their original purpose, you could map those to the new keycodes (the equivalent of pressing Fn+Fx) and go from there to get your volume keys and all the rest of the goodies that Apple hides up there.
Checkout this article form Wikipedia: http://en.wikipedia.org/wiki/Fn_key

Alternatively, the Keyboard settings pane of System Preferences will allow you switch whether you wanted a single keypress of any function key to send the keycode equivalent of Fn+Fx or simply Fx.   Perhaps you can use that checkbox to get what you want...  though I don't think it will work with your keyboard.  I think you need a newer keyboard to get this to work.