I would give her a go on some SpeediBeet. It's great stuff and easily chewed and digested for the older horse. I feed Speedibeet and Rice bran when I want to add weight, even on my comp horses. Rice bran is high in rice bran oil, and can be fed soaked in with speedibeet to make it like a mash. Nice and easy for the oldies to get down ;)
Mitavite gumnuts are very good for the older horse, but you'll have to look around at different options in your area and see what works best for her.
Instead of hay, you may have to feed chaff. If you can buy chaff in bulk, lucerne or wheaten are the best for weight gain, that would be excellent for her. Just poor it into a big feeding bin and she can eat away at that during the day instead of grass and hay.