I would caution against any corn products since they are high in starch & carbs, and possibly lead to founder issues. I'm in the process of putting weight on a hard keeper TB, and have done a TON of research in the process. The most helpful products I've found are:
Free choice hay - enough said.
Timothy Hay Cubes- I had started with Alfalfa-Timothy mix, but have since learned that the Alfalfa can sometimes increase the risk for founder and also, most horses don't need the amount of protein that comes with alfalfa. (about 14%). About $10 for a 50 lb bag
Beet Pulp - I'm using soaked beet pulp molasses free pellets. They blow up nice in about 15 minutes. These are very high in soluble fiber, very little chance of causing founder, colic, etc and can be used to substitute up to 50% of the horses forage diet without ill effects. About $12 for a 50 lb bag.
Stabilized Rice Bran. Rice bran in either powder or pellet form. It's great source of fat and can provide the extra calories you're looking for. Stabilized gives you a corrected calcium/phosphorus ration since rice bran itself is very high in phosphorus which can damage bone density over the long haul.
I'm currently feeding 2lbs hay cubes, 2lbs beet pulp and 1lbs of rice bran. I soak the beet pulp separate and dump out the water to rinse off any excess sugar that might be hanging on, mix it all together in a wet mash, and my TB loves it. I just started this regimen 3 weeks ago and am just starting to feel a little more meat between his ribs and so far he's gained 22 lbs.