Wow, you're not kidding about skinny. With her age, I would do anything very gradually. For forage, soaked beet pulp pellets make a nice mash. It would be easy for her to eat with her missing teeth and provides a highly digestable source of fiber. For added fat, maybe try adding rice bran in powder form to the beet pulp.
At her age, just being able to live out her days happy is really all you can ask for. Good luck. Glad to see someone's looking out for the senior horses out there.
Purina's Equine Senior has worked wonders for everyone I know. I use it too, but my mare is only 17. One of my friends has a gelding that is going on 43 this year who has been on soaked Equine Senior since he was 35. Still going strong!
If she's missing teeth, that makes it hard for her to grind her food properly and she can't get all the nutrients out of it that she needs.
Beet pulp is really good for packing on weight. There's also a liquid supplement called Red Cell that worked wonders for many horse I know that needed to gain weight. It's packed with vitamins and it's good for their coats, too. Make sure she's getting plenty of fiber and minimal sugar. You want a feed that has more oil and as little sugar as possible. A lot of senior feeds are real sugary, and I've found that regular pelleted feed works better. You could put a little bit of water in it to make it easier for her to chew. Soaking her hay might be a good idea, too. You might also want to try roasted soy beans. About a cup a day with her normal feed. They have a lot of oil in them, which is good for their coat and can help maintain weight. Horses love them, too. =D You want to make sure she gets plenty of pasture time, too. That's one of the best things you can do for her. Along with free choice hay.
I use the triple crown senior and it has made a HUGE difference on my guys. If her teeth are really bad then you can a little water to it, not a lot at first because she may not like the consistency. If her teeth are really bad she won't be able to eat hay... If she can I'd give her something that is really high quality. I give my old ones and my young one alfalfa, the others get orchard grass mix. I also give my older ones hay extender.
To be honost, for being 32 years old she doesnt look that bad, she's swayback from age and it adds to her skinny appearance. She's really, really thin but her coat is shiny and her eyes are bright. If you can get some weight on her, she'll be absolutely beautiful for an old gal... pretty mane and tail by the way.