There's no reason you can't say it if that is what you mean. But you should be aware that you would be saying something very strong. It would suggest a real problem with junk food that makes it difficult or impossible for this person to function well in daily life (like the effect that drug abuse has on a drug abuser). It is far stronger than just eating too much junk food as in more than is recommended for optimum health.

Yes, you could say that if you want to. Yo might also say 'overindulges' or 'really overdoes it' or 'slightly overdoes it' and so on.