I'm not sure how you would train a dog to go on a pad when they are already outside trained. Usually it goes the other way around.
Maybe try buying the kind of pad that has the scent that is supposed to attract the dog to pee on it. I would put the pad by the door most often used to go potty. They do sell liquid house breaking aids that have a hormone that attracts a dog to pee on it that might make the attraction stronger.
Since your dog already knows it is not supposed to pee inside it may be hesitant to pee there but if you finally get it to go there make sure you give a treat as a reward so your dog knows it is a good thing, |