is it possible to have hierarchy either in drop down by key or indrop down by index?
It should be similar to below one
Does abap web dynpro supports this kind of drop down?
Although you can't bind a hierarchy to the Drop Down you can kind of fake the display to the user and make it look that way. You would have to change the displayed values and put the indentation in yourself.
Otherwise you should consider that DDLB might not be the correct search paradigm. Perhaps a better approach would a self programmed value help. In the value help window you can of course use a tree or a table with a Hierarchy Column.