Can we add new sub rows in to appian Tree Grid

I've implemented an appian tree grid (https://docs.appian.com/suite/help/20.1/recipe-expand-collapse-rows-in-a-tree-grid.html)

I've a requirement to add main row and sub-row in to the tree grid. 

  

I could create a new main row eg: PR3 using addRowlink option. How can I add new sub row eg: Item 4 inside PR1 ?

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