Do you feed her at regular times? If so, she should toilet at about the same time every day. An hour after she eats, take her to her box and do not let her leave until she poops. When she poops, give her a huge reward.
If she poops somewhere and you don't catch her, make sure you clean it with an enzyme cleaner so she can't smell any remains.
Also, I have not used a litter box, but I've found that Thor is much better about pooping on a clean pad. I'm not thrilled about that, because it means I have to change his pads right after he pees, and then change one again five minutes later, but it's better than the alternative. I'm not sure if he actually prefers a clean space, or if the smell of urine prompts such a strong urge to mark that he can't think about pooping. I've wondered if that's why dogs take such a long time to find the perfect poop spot outside - maybe they are trying to find a totally odor-free area.
In any case, maybe set up a special "pood pad" somewhere that she only uses for poop.
Also, I think dogs can be crate trained at any age, but if you don't want to do that, you can either use the "umbilical cord" method, or get her an x-pen. Or just crate her after she eats. Again, take her out every fifteen minutes and put her on her pad. Give her five minutes to poop, and if she doesn't she goes back into the crate. She should learn pretty quickly that poop is the way to food and freedom.
The key to training yorkies is to not let them out of your sight during potty training. She may prefer to poop in private (many dogs do), or she may be vaguely aware that pooping in the house seems to upset you, so she wants to sneak off when she does it. You must be completely focused to break her of this habit. BUT, once she gets there... trust me, it's worth it!
