Yes, my Teddy was strictly outdoor trained to about 8 months. I re-trained him to use pads.
It's like starting with a new puppy. Limit Gunner's freedom to a smallish area (small room or xpen with the pad). Put him on the pad and use keywords and phrases to tell him to "potty here."
When he does it right, praise him and reward him. When he has the hang of it, give him more freedom. Teddy took to peeing right away on the pad, but pooping was more difficult. I had to corral him and really encourage him to do it. Praise and rewards are key.
Here is the blog I wrote on potty training:
Potty Training Small Dogs - Dog Life and Style