Not in core Q2A. Currently only 1 extra field is allowed and it's a text box. You could use that and ask them to put in "yes" or "no" if it's an adult question but that's a bit awkward.
An alternative would be to use tagging, and make sure all adult questions are tagged with a specific tag like "adult".
Then in the output_widget function of your plugin look at $qa_content['q_view']['raw']['tags'] for the current tags of the question.
