You can do core hack in page/register.php page at https://github.com/q2a/question2answer/blob/dev/qa-include/pages/register.php. At 150 line there is $qa_content['form']. For fields array you can add additional custom field. Then at 85 line, you can check value for that field by passing qa_post_text() to variable and compare value with if conditions.
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