Example filters to add or edit custom fields for the Citation Importer plugin.

citation-importer-filters.php

<?php
/*
Plugin Name: Citation Importer Filters
Plugin URI: http://stephanieleary.com/
Description: Add or edit fields for the Citation Importer.
Author: sillybean
Author URI: http://stephanieleary.com/
Version: 0.1
License: GPL version 2 or later - http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
*/

add_filter( 'citation_importer_fielddata', 'filter_citation_importer_source_fields', 10, 3 );

function filter_citation_importer_source_fields( $fields, $post, $item ) {
	
	$fields['source'] = sanitize_text_field( $item['container-title'][0] );
	
	if ( isset( $item['volume'] ) && !empty( $item['volume'] ) )
		$fields['volume'] = sanitize_text_field( $item['volume'] );
	if ( isset( $item['issue'] ) &&!empty( $item['issue'] ) )
		$fields['issue'] = sanitize_text_field( $item['issue'] );
	if ( isset( $item['page'] ) &&!empty( $item['page'] ) )
		$fields['page'] = sanitize_text_field( $item['page'] );
		
	return $fields;
}