See this thread:
2012 STI Stereo Build with OEM NAV H/U
I'm fairly sure it has a camera input. Don't know whether Subaru will offer an OEM camera for Imprezas in North America. But it should be fairly easy to add an aftermarket camera, just like on the 2008-2011 OEM Kenwood Navi HUs (except different plug on the 2012 nav).