I am trying to come up with a scenario -- is grandmother adding the food to a pile, or is something else intended?
yes, the meaning of this sentence is unclear. is grandmother bringing some extra food around to the house for me? or is she putting an extra serving on my plate?
Just a poor translation of "ditambah". Should be "increased" in this case, or at worst, "added to." Either way the meaning is awkward but what's important is to understand the grammar.
This could be used if your grandma (or a lady old enough to be your grandma) adds more food to your plate. My experience in Indonesia was that when people serve food to guests, they want the guest to eat it all. No matter how much you eat, they'll ask you to take more. And on some occasions, it was placed on my plate/bowl for me.
I don't understand the meaning of this sentence. It may be better to introduce "ditambah" in a context other than food.
The translation doesn't make much sense in English, but this is a very common use in Indonesia, actually. We'd probably say something like "Grandma gave me a second helping of food."
This translation really doesn't make sense - I cannot imagine a scenario where a native English speaker would say this.
Yeah, Blake is right. This is a common thing in Indo. ("Tambah lagi" is just an incessant refrain at anybody's house). Unfortunately it's hard to convey in the passive in English. At the very least, it should be "added to", or maybe "refilled".