Not having done the swap, I believe there are two "easy" options:
The first option, is the least favorable in my mind. It requires a special rear rotor that is a hybrid of the STI and the WRX so that the e-brake shoe fits the inner diamater.
The second option appears to be better, it uses a special e-brake pad that allows the use of standard STI rotors without problem. You should confirm that these will work for your WRX, as the term "classic" is open to interpretation: