How do I make my OTTB gain weight?

My six year old OTTB looks just like him, but chestnut. She's on one flake of alfalfa in the morning (big weight gainer but can make them hot) and 2 flakes of oat hay in the evening. I've been giving her beet pulp for about a week now and I'm gradually increasing it with 2 capfulls of corn oil, a capful of vinegar (for flies and healthy coat), a sprinkle of garlic (for flies), and starting next week she'll be gettinjg SmartPak, and a scoop of equine senior (which can also make them hot). My mare has the metabolism of a gerbil, so I know where you're coming from. Just be sure to stay away from too much sugars to reduce the risk of foundering (OTTB's don't typically have great feet). If you decide to get the beet pulp, which is recommended by a lot of people, try to stay away from the stuff mixed with moles.