Hi,
There is no option for that.
It would require to change the code of the file administrator/components/com_hikashop/types/user.php
You would need to change:
$query = 'SELECT user_id,user_email FROM '.hikashop_table('user');
to:
$query = 'SELECT a.user_id,b.username FROM '.hikashop_table('user').' AS a LEFT JOIN #__users AS b ON a.user_cms_id=b.id';
and:
$this->values[] = JHTML::_('select.option', (int)$user->user_id, $user->user_email. ' ' .$user->user_id );
to:
$this->values[] = JHTML::_('select.option', (int)$user->user_id, $user->username. ' ' .$user->user_id );