This is easy to create custom code pool in magento. Just follow the following step:

Open Mage.php that is located in directory app/Mage.php
Find include paths that was added as
$paths = array();

$paths[] = BP . DS . ‘app’ . DS . ‘code’ . DS . ‘local’;
$paths[] = BP . DS . ‘app’ . DS . ‘code’ . DS . ‘community’;
$paths[] = BP . DS . ‘app’ . DS . ‘code’ . DS . ‘core’;
$paths[] = BP . DS . ‘lib’;

Edit this section to:

$paths = array();

$paths[] = BP . DS . ‘app’ . DS . ‘code’ . DS . ‘custom’;
$paths[] = BP . DS . ‘app’ . DS . ‘code’ . DS . ‘local’;
$paths[] = BP . DS . ‘app’ . DS . ‘code’ . DS . ‘community’;
$paths[] = BP . DS . ‘app’ . DS . ‘code’ . DS . ‘core’;
$paths[] = BP . DS . ‘lib’;