Use this code snippet to make sure that a number field value from 0-9 is saved as 00-09.

add_filter('frm_prepare_data_before_db', 'save_field_as', 10, 2);
function save_field_as( $value, $field_id ) {
	$target_field_id = 122;
	if ( $target_field_id !== (int) $field_id ) {
		return $value;
	}
	if ( is_numeric( $value ) && 1 === strlen( $value ) ) {
	$value = '0' . $value;
}

	return $value;
}