Access first cell first row (expander_placeholder) in nested table

[didoshotev]

Is there a way accessing the first cell in the first row (as shown below) while creating the table in order to put a button there ?
I see it is initialized by default and it has some property and methods defined which can be accessed after it has already been rendered.

That's headerGroup object and the first object is the one I need to access while creating the data, the other ones are dynamically created based on external data.

That's exactly the cell I need to access and customize: