How heavy are your bales? Does she also have grazing time? If she doesn't have much grass or grazing, and your bales are normal 55-65 lb grass hay, then she's really not getting enough hay. A horse should get at least 2% of its body weight a day in forage and/or hay. That's roughly 20 lbs for a 1,000 lb horse. If a 65 lb bale is lasting you a week, then she's getting less than 10 lbs hay a day. That's half the requirement for a horse much smaller than she is.
To put weight on my 15.2h Anglo Arabian gelding, he was getting 30 lbs of hay a day, so one bale lasted just two days (he didn't have any real grazing to speak of).
For quality calories, I would go with hay pellets over beet pulp. If you're feeding grass hay or Timothy, then Alfalfa pellets are good. If you're feeding Alfalfa or mixed Alfalfa/grass hay, then Timothy pellets or Alfalfa/Bermuda pellets would be best. Hay pellets will give your horse real calories, nutrients, and in the case of Alfalfa pellets, amino acids. Amino acids will help her build real muscle weight, not just put on fat. You can give 3-5 lbs a day of hay pellets as a supplement to forage/hay.
I would also have her on milled flax. 4 oz (by weight) a day for weight gain, 2 oz for maintenance. I use Nutra Flax and have been very happy with it (shipping is free). They will custom blend ingredients for you at the factory. You might ask about adding Yeast and Probiotics, both aid in digestion and weight gain. Omega Horseshine is also a good product.
Check page 2 of this article on good tips for skinny horses.
MyHorse.com - Let Your Horse Eat Hay