Treegrid - display expand/collapse arrow

How can i force the treegrid to display the expand/collapse arrow on each row even thought i haven’t added child rows to the very row.

The reason i am asking this is because i have streaming data from server and at any given moment there may come array with 10 elements each of which contains up to 100+ child rows for the grid and using the add method is veeeeeeeeeeeeeeeeery slowly and the UI freezes. So in case i get some data from the server for a particular row i want to show the expand/collapse arrow and when the user clicks on it to insert the childs.