Using USB-Keyboard (connected my mbp) to control IPad via Bluetooth Connection
It is not currently possible to control a non-jailbroken iPad in the way you've described.

Out of curiosity, is there a reason it has to be an external USB keyboard and not the keyboard built into the MacBook Pro itself?

Perhaps if you mention why you'd like this functionality (e.g. game control? typing into a notepad app?), someone could propose an alternative.  The Ask Different users can be quite creative when questions are worded broadly enough.  :]