Can you potty train an old dog?

My mom has my chiuahuah that is about 6 years old. He's not really potty trained. He, and my other dog are in my mom's garage, in a big pen. They go outside all the time. but at night, and durring the day they are in their pen.
They know they are supposed to potty on newspaper, but if you let them in the house, they will potty everywhere.

I was wondering if I could bring him to my house and potty train him. I dont mind him going on paper, but I dont want him to poo in the house.
Has anyone ever potty trained an older dog??

 

That particular breed is tough to potty train to begin with. They get scared of everything because they are so small. And they know it. Otherwise, it can be done. With a KIND eye, and touch. Show him/her what you expect. Reward when it does what you want.