WP All Import - Redirection AddOn (http://www.wpallimport.com/ - https://wordpress.org/plugins/redirection/)

add-redirect.php

<?php

/*
Plugin Name: WP All Import - Redirection AddOn
Description: Add a redirect for each post imported.
*/
// If this file is called directly, abort.
if ( ! defined( 'WPINC' ) ) {
	die;
}


/**
 * Create a redirect.
 *
 * @param string $source
 * @param string $target
 * @param string $group_name
 */
function create_redirect( $source, $target, $group_name = 'migration' ) {
  	// Get the correct group ID, group will need to exist.
	global $wpdb;
	$group_id = $wpdb->get_var( "SELECT id FROM {$wpdb->prefix}redirection_groups WHERE name='{$group_name}'" );
  	// Check if a redirect for this URL already exists.
	$exists = Red_Item::get_for_url( $source );
	if ( empty( $exists ) && !empty( $group_id ) ) {
    	// Create the redirection.
		$ret = Red_Item::create( [
			'url' => $source,
			'action_data' => [
				'url' => $target,
			],
			'regex'  => 0,
			'group_id'  => $group_id,
			'match_type'  => 'url',
			'action_type' => 'url',
			'action_code' => 301,
		] );
	}
}


/**
 * Set a redirection if any.
 * @param object $post
 */
function set_redirection( $post ) {
  	// Prepare the source.
	$source = get_post_meta( $post->ID, '_legacy_path', true );
	$source = wp_parse_url( $source, PHP_URL_PATH );

	// Prepare the target.
	$permalink = get_permalink( $post->ID );
	$target = wp_parse_url( $permalink, PHP_URL_PATH );
	
  	// Create a redirect.
	if ( $source && $target ) {
		create_redirect( $source, $target );
	}
}


/**
 * Register actions to run after each saved post.
 * @param integer $id 
 */
function redirect_after_saved_post( $id ) {
	$post = get_post( $id );
  	// Handle redirection.
	set_redirection( $post );
}
add_action( 'pmxi_saved_post', 'redirect_after_saved_post', 10, 1 );