I came across this, which seems to be the solution:
$fields['billing']['billing_options'] = array(
'label' => __('Options', 'woocommerce'),
'placeholder' => _x('', 'placeholder', 'woocommerce'),
'required' => false,
'clear' => false,
'type' => 'select',
'options' => array(
'option_a' => __('option a', 'woocommerce' ),
'option_b' => __('option b', 'woocommerce' )
)
);
Now I’m wondering how you can add the chosen css styles to these new options to match the other drop downs… anyone??