Yes, you can do that very easily. I first subscribed to the PS/Lr deal, while still using the CS6 master collection for the other apps. Then a few months later I upgraded to a full CC subscription.
I did it through chat, and the whole deal was up and running and functional inside ten minutes. All I had to do was sign out of the CC app and back in, and then it was all there. What they did was just cancel my existing subscription and sign me up for a new one. It didn't cost me anything extra.