Pre Populate a Tree Grid Column to a property value of base Tree

We have implepemted a Tree Grid which uses a Nested Page List structure.

There is a Add Button on click of which we have configured action as Add Item(After). This button can be used to add Outer PageList (By selecting the outer list and clicking on Add) and Inner Page List(By selecting one of branch and clicking on Add)

Problem: We need to prepopulate one of the Inner Page List property(text) with a OuterPageList property on click of add.

Kindly let me know if any1 has any solutions in this regard.

Thanks in Advance

Ashish