Yes, I have added subs to my 2012 focus, however it is an SE and without the sony system. The sony system is going to cause you much more problems than the regular 4-6 speaker configuration because of the built in amp/sub. You could just splice in an LOC and run that to add additional subs to run aside the built in sub, however you will want to be careful how much strain you are putting onto the alternator after doing this.
This is why it is not really recommended to adding aftermarket subs to the sony system, or any system with amps/subs added in by stock. You have to fully know what you are doing and what to order not to cause problems with the stock configuration. The last thing you want to do is add subs to it not knowing what your doing and cause hundreds/thousands of dollars of damage to a no longer warrantied sound system.