杂粮饭 za liang fan It can be used to name all staple food which is rice mixed with other cereals.

I'm fairly sure MILLET should be used in the singular. Some food items are used this way, just like one doesn't say RICES.

Dan is right that "millet" should be used in the singular only (whatever Wikipedia says!). However, "millet" is a particular type of grain, so you could only say "rice with millet" if you specifically mean "millet". If you just want to say it's a mixture of grains, then "rice mixed with other cereals" is what you need.

There's not a special word for it that I know of in American English. You could either just call it rice, or you could mention the other ingredients the same way you did in your question.

Of course you could call it "rice with millets" if you like. But to be honest, I had to look up "millets." I'm not sure if I'd ever heard the term before. Your listener would be likely to ask you what it means.