You will need to edit /includes/templates/YOUR_TEMPLATE/templates/tpl_modules_create_account.php

Just find this code:
Code:
<?php echo zen_draw_input_field('extrafield', '', 'id="extrafield"') . (zen_not_null(ENTRY_EXTRAFIELD_TEXT) ? '<span class="alert">' . ENTRY_EXTRAFIELD_TEXT . '</span>': ''); ?>

And replace with this:
Code:
<?php echo zen_draw_radio_field('extrafield', 'yes', '', 'id="extrafield-yes"') . '<label class="radioButtonLabel" for="extrafield-yes">Yes</label>' . zen_draw_radio_field('extrafield', 'no', '', 'id="extrafield-no"') . '<label class="radioButtonLabel" for="extrafield-no">No</label>' . (zen_not_null(ENTRY_EXTRAFIELD_TEXT) ? '<span class="alert">' . ENTRY_EXTRAFIELD_TEXT . '</span>': ''); ?>
I did not test it, but I *think* it should work.

You can do a similar edit in the tpl_account_edit_default.php file, so your customers see the same radio button on their My Account page.