Learn Hardcore Malay: Dia suka makan.

Yes, dia can mean either "he" or "she" (and sometimes even "they") depending on the context. There is no separate pronoun to indicate male or female.

Can I say Dia suka makan nasi to be more specific about what the person likes to eat?

Absolutely! Adding nasi (rice) or any other dish makes the sentence more specific, e.g., Dia suka makan nasi ("He/She likes to eat rice"). You can replace nasi with any food you want to mention.

Why is there no word like "to" before makan?

Malay does not use a marker like "to" before verbs. You just use the base form of the verb. So Dia suka makan literally goes straight from "like" to "eat."

How do I emphasize that the person really likes eating?

You can add the intensifier sangat ("very") or memang ("indeed/really") for emphasis, e.g., Dia sangat suka makan or Dia memang suka makan.

Do I need to change dia if I'm talking about multiple people?