Sure, though it's not clear what you mean by "turn it into" a USB I/O card. It has USB, it has I/O's. Do you want:
current protection?
high current source/sink capability?
built-in software and/or PC drivers?
The software that comes with our Gator board has a built-in I/O application that lets you play with the I/O lines directly, as well as an Analog application that lets you look at and record the analog inputs. You could probably write something similar for the Arduino Mega without much effort.
The arduino can easily operate as a USB I/O device but it's not a direct USB device as you will have to use it as a serial device.
The processing language works well with the Arduino. All you really need to do is have the Arduino send the I/O and/or analog data over the serial link to processing and have it do what you want.
I had an I/O pin on the arduino send that data to processing which played a bell sound. It could have done any number of things.