Well, the commercial feed you are buying is already mostly grain. Adding 'natural' grains will do nothing more than lowering the nutrition in the feed.My plan was to substitute a percentage of the commercial feed we buy with natural grains. Depending on the type of grain, it contains many vitamins and minerals already.
I think it is important to remember that chickens are omnivores and not strictly grain eaters.
Vegetative sources of protein, i.e. grain, is deficient in several essential amino acids like lysine, methionine, cystine and tryptophan. The only way to make a complete vegetarian feed that provides the protein chickens need is by a careful balancing of grains and legumes as well as adding at least two essential amino acids in synthetic form.
As for vitamins and minerals, yes grain has them but not all those that chickens need.
If you want to do this, it may help to also include something like fish or porcine meal to supplement some of those amino acids and the vitamins, minerals and fats missing in the grain.
I just don't think that substituting feed with what is already the main ingredients in feed is going to save money in the long run.