Is that Purina Equine Senior? If so, 1 lb a day is really doing almost no good- their feeding directions for a 1000 lb horse not in work is 13.5 lbs daily, so that's the minimum you would need to feed daily to fulfill nutritional requirements. I'd look into replacing it with a ration balancer (which is designed to be fed at low rates of 1-2 lbs/daily), or just cutting it out completely. Purina makes two ration balancers- Enrich 32 for grass diets, and Enrich 12 for alfalfa diets. You'd want Enrich 12 since your horse is on an alfalfa only diet.
I've seen it mentioned a couple times that a horse on a straight alfalfa diet is getting pretty much everything they need nutritionally, but I haven't been able to find any studies on it, so I'm still not quite sure that's true, especially given that the nutritional value of hay is strongly dependent on the nutrient value of soil it was grown in. Additionally, hay loses nutritional value the longer it is stored.
Aside from alfalfa, I really like beet pulp and rice bran for putting weight on. Beet pulp needs to be soaked into a mash, but it can be fed safely in large quantities and is much healthier than feeding large quantities of grain.
Rice bran has a high fat content, which makes it great for weight gain/maintenance, but it also spoils quickly and has an inverted calcium-to-phosphorous ratio. You can get rice bran that is stabilized to increase the shelf life, and I wouldn't recommend buy any rice bran that hasn't been stabilized since it can spoil before it even gets sold by the feed store. You can also buy rice bran that has been fortified (calcium added to offset the high phosphorous), but given that you're feeding 35 lbs of alfalfa daily, it's really unnecessary. You generally want to limit the amount of rice bran to a few pounds daily.
I don't personally see much value in feeding oats, though I know many people feel otherwise.
Whole flax seed or oil is another great feed. Whole flax seed is about 8% fat (the oil is 100% fat) but the fat is very healthy type of fat, high in omega-3. This is especially important for horses that don't get fresh grass as their primary forage, since fresh grass has good levels of omega-3 that quickly break down when dried. Omega-3 is an anti-inflammatory fatty acid, while omega-6 promotes inflammation, so you'd definitely want to keep them balanced in a horse with a problem like arthritis.