Is there a reason that you cannot use the dual-CSV format, which supports dual-response none?
Behind the scenes, when we code the tasks for estimation there are three scenarios:
'Would Choose' - The task is written normally with a none option.
'Would Not Choose' - A first task is written without the none, and a second one with the none included and chosen (this is analagous to what you describe).
'Missing (no response available)' - A task is written without the none option.
This can be coded in the single CSV as you suggest, however you will need to handle the none specially as tasks cannot mix none options within the same file. To do this, you need to add a new column as a 'None' attribute. Your none concepts will have a 1 for that column (and a 0 in all other attribute columns). All other concepts will have a 0 in the None column. In HB, you need to code that column as User-Specified and the project should specify there is no none option. The new column will react semantically the same as the none option.