I inquired about this recently. The answer is, basically, NO. You either buy a whole new license to CS6 extended, or, if you have CS5, you can buy an upgrade to CS6 Extended for some $400 or $500 (I forget the exact price).
It cost me NZ$800 to upgrade from CS5 Extended, to CS6 Extended. Fortunately we have escaped the vicious Adobe Price Gouging in NZ with CC, but I will never forgive them for ripping me off all those years with previous versions. I have paid Adobe thousands of dollars more than than its North American customers with all the products and upgrades I have bought over the years, and it had nothing to do with exchange rates and local duty/tax.
If you live in a country that still suffers this grossly unfair practice, get organized, be vocal, get your MP and national government involved. That's what we did in Australia (MP Ed Hussic) and New Zealand, and I now pay pretty much exactly the same for my CC subscription, as customers in North America.
You should normally have a 30 day return if you bought it from Adobe, then you can purchase the Extended version.
Customer Service should be able to help you on this.