What is your "alrounder" pellet? It sounds as though it should already be properly balanced for the nutrient requirements of a horse, either at light to moderate work unless stated otherwise. I would use extreme caution when feeding both the pollard and bran, especially in high volumes exceeding 2 lbs, because they do contain an inverted calcium-to-phosphorous ratio (ie: more phosphorous than calcium). Pollard can be an excellent source of calories and does indeed help in some cases with weight gain. I'm not certain though about bran improving digestion.
As for the vegetable oil you may be able to top dress some but keep in mind that your horse will also be getting fat calories via the pollard. If you're looking to maintain weight with a good, digestible fiber source I would recommend looking into some beet pulp.