How can I add actor elements to an array in Datatable?

Hi, I built a structure named DemoStructure and add an empty actor array in it. Then I built a datatable with DemoStructure then I found I can not add any element to actor array. I thought all actors I wish to add to actor arrays are the children of actor class, but I can not add them to an actor array. I wonder if there is a way to make an array in datatable and then adding different child actor elements to it?
In addition, if I add elements to actor array in structure, why can I only add actors from current level?

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