Is the purpose to save money or provide better nutrition?
You can mix some of that stuff in a pinch but IMHO, you'll still be missing some essential nutrients. Some vitamins, minerals and amino acids are inevitably going to be lacking.
I've looked into it many times and determined it would be difficult to get things like selenium, zinc, copper vitamin D3 etc. in the right quantities and it definitely is more expensive.
The main reason is, feed manufacturers are buying grains and legumes by the trainload and other ingredients like vitamins and minerals in 100 lb. to ton quantities. A 50 lb. bag of barley is more expensive than a 50 lb. bag of starter feed.