Well most feeds will do that if fed in quantities that supply more calories than the horse will use up for daily maintenance, work etc!
The cheapest way to add condition (apart from grazing) is usually to give the horse as much hay or haylage as it will possibily eat. Once the horse is on adlib forage, you can add bucket feed - but what is best for your horse depends upon whether it has any feed intolerances, specific ailments such as azoturia, ulcers, laminitis, and whether certain feeds affect his performance/temperament, also whether the horse has a poor appetite and/or doesn't like particular feeds.
For my veteran poor doer I have found the best diet is adlib hay, sugar beet, veg oil and some micronized flaked barley, but that diet will not suit all horses!
weight gain from allen page and fibre beet. you will have to tell us more about your horse o give you more ideas? like how old what work he is in. does he live out or in how big he his that kind of stuff?