Sure, it will work, but the car will be a mess. FWD + Auto tranny + low compression motor = nasty drive.
You might think on buying a complete drivetrain from japan including the stuff to convert to AWD. I got a complete STi V1 setup w the entire driveline for cheap. The only part you might have to get is a fuel tank if Subaru used different ones in the FWD impreza compared to the AWD version.