Determine what is in your car before thinking about swaps, which isn't likely to happen anyway.
4 ways to tell:
1. look in the cabin, does the gear selector move only straight forward and back and have a button on it and the base plate says P-N-D-1-2-3-R or some simlar combination for Park, neutral, drive..etc? That's an automatic. If it's a loan stick that has play left/rigth as much as forward/back without those demarcations then it's a manual.
2. Post a pic of the gear selector/center console, and we can tell you or compare to online photos of the same
3. plug the VIN into an online VIN decoder and it'll tell you what transmission it has.
4. Auto's have a long dipstick under the brake master cylinder on the drivers side, and a front diff gear oil cap on the passengers side. Manual only has gear oil cap on passengers side.
This is too funny not to mention - I'm not sure how to own a car, see the trans fluid, and pull the engine without knowing what kind of transmission it has, the process to remove the engine is slightly different for each.
A 4 speed easily installs in place of a 5 speed, they're like lego's and it bolts right up, but getting it to run is mad money or labor that's out of 99.9% of people's time or money thresholds. Wiring, TCU, cruise, cables, pedal assembly, driveshaft length differences, center console.... lots of work and parts. Yes other manual transmissions in other subaru's will fit that vehicle. if they have a different gear ratio you'll need a matching rear diff and your speedo will be off by a litlte bit.