I think it is harder once a dog is already outdoor potty trained to re-learn to do both. Because they've always been taught going inside = bad. With that said, nothing is impossible and I think it can certainly be trained. Dogs are smarter than we give them credit for.
Jackson likes the cold and the snow however a few of our storms were like impossible to even walk out the door. And I would BEG him to go in the garage or on the porch -- he rarely would do it. The porch he would because it had about a foot of snow inside of it (even though it was screened in, lol) so that was nice. But I was never able to get him to go on the pad when I briefly tried as a pup. But I'm happy with him being outdoor trained so I just deal with the winter.