Is your objective to actually improve the sound of the stereo, or do you just want "doof doof" bass?
If you actually want it to sound better, you could probably start by adding Dynamat (or similar) in the doors, and upgrading the speakers themselves.
It would also depend on which audio system it has, there are a few different ones of varying quality.
Running any sort of amp off the stock head unit will be tricky as it doesn't have RCA outputs, some amps are available with speaker level inputs but wiring could get a bit messy. You'd also have to work out where to actually position the sub, unless you're happy to lose half of the boot space.