I just figure out how to do it with a bit of reverse engineering, hopefuly this helps someone :
$availability = array();
$availability[0]['type'] = 'time:range';
$availability[0]['bookable'] = 'yes';
//Default priority
$availability[0]['priority'] = 10;
//case 'time:range'
$availability[0]['from'] = wc_booking_sanitize_time( $hours );
$availability[0]['to'] = wc_booking_sanitize_time( $hours );
$availability[0]['from_date'] = wc_clean( $date );
$availability[0]['to_date'] = wc_clean( $date );
add_post_meta( $product_ID, '_wc_booking_availability', $availability );