Adding a RAM

You will be able to do that, as long as you can find another stick of the exact model of the RAM of the current one. RAMs are not meant to be mix and matched, so I would advise you to purchase a new dual channel module if you can't find the exact same one, it's better than buying a random one and hope your PC can stay on for longer than 30 minutes.
 


You can try, nothing is going to blow up. However, I can pretty much guarantee that it won't work for long if you put 2 sticks of different capacity in at the same time.