What you have asked is not possible as the line item has been formatted as the L3 list. WIthin a composite hierarchy each list is separate but connected via its relationship as parent and child. Therefore, when you format the line item as one of these lists you are only providing access to that single list.
However, you can create a fourth list which contains all children and parents within a flat list. Format your line item with this and allow users to select from the full range of options.
The challenge here comes from when you want to use the selection to inform downstream calculations.
Firstly, you will need to map each item of the new list as a dimension within a mapping module to each item of the corresponding hierarchy. To achieve this you will need to use a line item for each level and format each with the corresponding list from within the hierarchy.
Secondly, you will need a series of nested IF...THEN functions to cycle through a series of algorithms so that you can direct any calculation to the correct line item in the mapping so you can map back to the correct level in the source.
A simpler solution would be to create a module for for each level, calculate the outcome and enable the user to select the final outcome to take further into the process.