It's not a question of whether or not Apple can add a discrete GPU. Of course they could add one, just as they could rifle handgrips for gamers. The question is whether or not Apple will add a discrete GPU.
The whole point of Iris Pro 5200 HD graphics, which Intel developed largely for Apple, is to obviate the need for a discrete GPU by providing good integrated graphics performance. So, if Apple were to include a discrete GPU in the 15" Haswell MBPs (which I think would be an insane business decision and I would sell my AAPL stock if they did it), then it would be paired with a Haswell GPU with weaker HD 4600 integrated graphics.
Intel HD 4600 plus a discrete GPU would, in my opinion, be crazy. Iris Pro 5200 plus a discrete GPU would be double crazy.