Sure you can add one, but how do you plan to tap the beer from the kegs?
If you are going to bild a kegerator or keezer you should add an extra faucet when you add an extra keg. If you just us a picknicktap you should buy an extra picknicktap.
On the other hand, it is no problem to have more kegs than faucets. You can build an stock on filled kegs and change the keg whenever you like, since you can easily disconnect a keg halfway.