This will be a very expensive undertaking.
There are many things to take into account, first of all mechanically it's easy, making it actually work will be the trick.
@Scoobs you say the new WRX and STI use different engines so the transmission work work, not entirely true. Subaru builds cars like Lego, you can match almost any engine to any transmission throughout the range. No adapters needed.
A newer WRX has an FA20 engine, the outback has an FA24, the CVT in the WRX is near the same as the outback high torque, being that a WRX still comes with a manual option, mating the transmission to an outback engine should be relatively straight forward.
You would need the transmission, axles, drive shaft, rear diff, shifter, shifter cables, flywheel, clutch, release bearing, clutch fork, clutch slave cylinder, hose, master cylinder, clutch pedal assembly, and thats just the mechanical things!
Electrically it would be easier to get rid of the original ECM, find a WRX and use the wiring from that, which would then render the eyesight and other safety features useless, it MAY be possible to basically change the outback wiring to WRX but then you would still get a bunch of warning lights and lack of communication from the media hub and combination meter which would be a nightmare to get rid of.
Or you go custom harness and use a Haltech ECM.
Either way ots going to be extremely expensive, just for you to be able to shift your own gears!