I recently had a similar need for more I/O pins ;-)
You can do it, but you must disable the UART the 'hard way'. It is activated by default and overrides the pin-direction (input/output) internally in hardware. Once you've disabled it you cannot use any hardware serial communication anymore (in that program).
Insert this at the very beginning of setup()
UCSR0A = 0x00;
UCSR0B = 0x00;
UCSR0C = 0x03;
This sets the uart control & status registers to their default values and turns the uart + port override off.
At the end of the arduino init() function it does this.