You have a big heart, and I'm sure we would all do the same if we saw a little 5wk yorkie girl (and the guy is ready to rehome them!!!)! I know I would!
Best of luck with your baby girl. Just keep her warm, well-fed, and in safe areas in your home (so she won't get underfoot!) Also have you heard about coconut oil? Lot of premie baby puppy milk contains coconut oil because it contains fatty acids same as mothers milk! I would give her a little lick every day (@ 1/8 tsp). Helps build a healthy immune system!! I love and use CocoTherapy. They're USDA organic and the best for pets! Check this out (see the blue box on the left where it talks about coconut oil in puppy milk:
CocoTherapy - Amazing Coconut Oil