What can I feed my 10 old yorkie to help her gain weight?

Has she been checked out by a vet?
10 years is getting on for a Yorkie so she needs a full check-up.

If all is well there then I would recommend a dry food specifically designed for elderly dogs before trying any home-made diets.

A really good senior dog food would be the Innova brand dog food. They have two different formulas, one is for an active senior, the other is regular. You may or may not have to soften the food in warm water. The kibbles are bite-sized and crumble in the mouth easily.

Its expensive, but well worth it!

Innova Natural Dog Food, Cat Food & Healthy Dog Treats – Natura Pet Products

You may have to copy and paste.

And I wouldn't talk to a vet about dog food. They always seem to want to recommend Science Diet, and Science Diet is nothing be ground yellow corn and a lot of fillers. If your dog is in good health, you won't be needing any of the prescription foods. The Innova is a holistic food that has never been on the recall. Its great stuff. My dogs are on it and they are thriving! I swear by this food!

I wouldn't talk to a vet about food either, mine is also an advocate for Science Diet and the food is rubbish, they are paid to stock it in their clinic.

The dog still needs a vet check though.