Call with $this->EE->load->library('my_ftp'); It will load CI_ftp and replace $this->EE->ftp with your ftp class

EE_ftp.php

<?php  if ( ! defined('BASEPATH')) exit('No direct script access allowed');

require_once(BASEPATH . 'libraries/Ftp.php');

class EE_FTP extends CI_FTP {

	var $timeout = 90;

	/**
	 * FTP Connect
	 *
	 * @access  public
	 * @param   array    the connection values
	 * @return  bool
	 */
	function connect($config = array())
	{
		if (count($config) > 0)
		{
			$this->initialize($config);
		}

		echo 'hello';
		exit();
		if (FALSE === ($this->conn_id = @ftp_connect($this->hostname, $this->port, $this->timeout)))
		{
			if ($this->debug == TRUE)
			{
				$this->_error('ftp_unable_to_connect');
			}
			return FALSE;
		}

		if ( ! $this->_login())
		{
			if ($this->debug == TRUE)
			{
				$this->_error('ftp_unable_to_login');
			}
			return FALSE;
		}

		// Set passive mode if needed
		if ($this->passive == TRUE)
		{
			ftp_pasv($this->conn_id, TRUE);
		}

		return TRUE;
	}
}

MY_ftp.php

<?php  if ( ! defined('BASEPATH')) exit('No direct script access allowed');


// # ====================================
// # Load ftp and overload the property

class MY_FTP {

	public $CI_ftp = null;
	public $MY_ftp = null;

	public function __construct($config = array())
	{
		$this->EE =& get_instance();

		if ($this->MY_ftp === null)
		{
			// load both libraries
			$this->EE->load->library('ftp');
 			$this->EE->load->library('EE_ftp');

 			// shuffle some variable
			$this->CI_ftp = $this->EE->ftp; // keep a copy
			$this->MY_ftp = new EE_FTP($config);
			$this->EE->ftp = $this->MY_ftp;
		} 
	}
}