The tricky thing is the table on the edit assign supports right click and can pop up a menu. The menu is the magic.
From the menu, select the insertMissingToData, then the array setting will work.
The change to the element is
This is not the only tricky thing as it is not only apply to the variable in an array, for any variable is same if the XML document is not initialized by some operation already. You have to set insertMissingToData="yes".
Another tricky thing is the generated XML. The order of XML elements is related with the order of assigning.