It is possible to train an adult yorkie. An old dog can learn new tricks.
I got Tatiana when she was three years old. She had lived with her breeder for the first three years of her life, so living with me was something completely new to her.
Do you happen to know your yorkie's poop schedule? With Tatiana, I noticed she likes to poop first thing in the morning when we get up, or right after her meals. I have her potty mat inside her exercise pen. The first thing I do when I get up is put her in her exercise pen (and close the gate for a few minutes). Sometimes she poops. Also, I feed her in her exercise pen. After she eats, she goes to the potty mat, and pees and poops. When she's done eating and pottying, I open the gate of the exercise pen, and she walks out. I leave the gate open at all times (when she's not inside for her meals and potty breaks). Guess what? She takes herself into the exercise pen when she needs to potty, and goes on the potty mat. It's become a habit for her.
This works for Tatiana. I hope it's helpful to you.