A custoim importer for WordPress

class-custom_import.php

<?php
/*
Plugin Name: Custom Importer
Plugin URI:
Description: Import posts or not
Author: Ralf Albert
Author URI: http://yoda.neun12.de
Version: 0.1
Text Domain: custom-importer
License: GPL version 2 or later - http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
*/

namespace RalfAlbert\CustomImporter;

// check if we come from admin.php?import=...
if( ! defined( 'WP_LOAD_IMPORTERS' ) )
  return;

// Load Importer API (Importer API = Backendpage admin.php?import=...)
require_once ABSPATH . 'wp-admin/includes/import.php';

if ( ! class_exists( 'WP_Importer' ) ) {
	$class_wp_importer = ABSPATH . 'wp-admin/includes/class-wp-importer.php';
	if ( file_exists( $class_wp_importer ) )
		require $class_wp_importer;
}

function custom_importer_init() {
	load_plugin_textdomain( 'custom-importer', false, dirname( plugin_basename( __FILE__ ) ) . '/languages' );

	register_importer(
		'custom',
		'CustomPress',
		__('Import <strong>posts</strong> or not.', 'custom-importer'),
		array(
			new Custom_Import(),
			'dispatch'
			)
	);
}

add_action( 'admin_init', __NAMESPACE__ . '\custom_importer_init' );

class Custom_Import
{

	public function dispatch() {

		$this->header();

		$step = filter_input( INPUT_GET, 'step', FILTER_SANITIZE_NUMBER_INT );
		if( empty( $step ) )
			$step = 0;

		switch( $step ) {

			// start import
			case 0:
			default:
				$this->greet();
				$this->enter_url();
				break;

			case 1:
				$this->read_xml();
				$this->parse_xml();
				$this->sanitize_data();
				$this->insert_data();
				break;

			case 2:
				$this->finish();
				break;
		}

		$this->footer();

	}

	// Display import page title
	public function header() {
		echo '<div class="wrap">';
		screen_icon();
		echo '<h2>' . __( 'Import CustomPress', 'custom-importer' ) . '</h2>';
	}

	// Close div.wrap
	public function footer() {
		echo '</div>';
	}

	/**
	 * Display introductory text and blah
	 */
	public function greet() {
		echo '<div class="narrow">';
		echo '<p>'.__( 'Howdy!', 'custom-importer' ).'</p>';
		echo '</div>';
	}

	public function enter_url() {

		echo 'Display input form for url';
		/*
		 * Displaying a input form to enter an url
		 * The form data should be send to the current page and
		 * an additional query argument 'action=1' to anter the next step
		 */

		$this->redirect( 1 );

		return TRUE;
	}

	protected function read_xml() {
		echo '<ol>';
		echo '<li>Read XML...</li>';
	}

	protected function parse_xml() {
		echo '<li>Parsing XML...</li>';
	}

	protected function sanitize_data() {
		echo '<li>Sanitizing data...</li>';
	}

	protected function insert_data() {
		echo '<li>Inserting data...</li>';
		echo '</ol>';

		$this->redirect( 2 );
	}

	protected function finish() {
		echo '<p>All done!</p>';
	}

	protected function redirect( $step = 0 ) {

		$url = home_url( add_query_arg( array( 'step' => $step) ) );
		printf( '<p><a href="%s">Next step...</a></p>', $url );

		return TRUE;

	}

Referenced on WordPress StackExchange.