from http://eventespresso.com/topic/export-custom-fields/ You can add this code to a functions plugin or into your WordPress theme's functions.php file. More info here: http://eventespresso.com/wiki/create-site-specific-plugin-wordpress-site/

include_custom_fields_csv.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_filter(
    'FHEE__EventEspressoBatchRequest__JobHandlers__RegistrationsReport__reg_csv_array',
    'espresso_add_meta_value_csv',
    10,
    2
);
function espresso_add_meta_value_csv( $reg_csv_array, $reg_row ) {
    // get the event's ID
    $event_id = $reg_row[ 'Registration.EVT_ID' ];
    // example of getting the post meta
    $room_number = get_post_meta( $event_id, 'room_number', true );
    if ( $room_number != '' ) {
        $reg_csv_array[ 'Room Number' ] = $room_number;        
    }
    return $reg_csv_array;
}

include_paypal_id_csv_example.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file
add_filter(
    'FHEE__EventEspressoBatchRequest__JobHandlers__RegistrationsReport__reg_csv_array',
    'espresso_add_paypal_id_csv',
    10,
    2
);
function espresso_add_paypal_id_csv( $reg_csv_array, $reg_row ) {
    // get the event's ID
    $event_id = $reg_row[ 'Registration.EVT_ID' ];
    // example of getting the post meta
    $paypal_id = get_post_meta( $event_id, 'paypal_item_id', true );
    if ( $paypal_id != '' ) {
        $reg_csv_array[ 'PayPal ID' ] = $paypal_id;        
    }
    return $reg_csv_array;
}