Hack Woocommerce to add product custom fields

woocommerce_template_functions.php

  <form action="<?php echo esc_url( $_product->add_to_cart_url() ); ?>" class="variations_form cart" method="post">
                        <table class="variations" cellspacing="0">
                                <tbody>
                                <?php foreach ($attributes as $name => $options) :?>
                                        <tr>
                                                <td><label for="<?php echo sanitize_title($name); ?>"><?php echo $woocommerce->attribute_label($name); ?></label></td>

// Start custom code to add custom text field
                                                <?php if(is_array($options) && $options[0] == "CUSTOM_TEXT") : ?>
                                                <td>    <input type="text" id="<?php echo esc_attr( sanitize_title($name) ); ?>" name="attribute_<?php echo sanitize_title($name); ?>"/></td>
                                                <?php else: ?>
                                                <td><select id="<?php echo esc_attr( sanitize_title($name) ); ?>" name="attribute_<?php echo sanitize_title($name); ?>">
                                                        <option value=""><?php echo __('Choose an option', 'woothemes') ?>&hellip;</option>
                                                        <?php if(is_array($options)) : ?>
                                                                <?php foreach ($options as $option) :
                                                                        $option_term = get_term_by('slug', $option, $name);
                                                                        if (!is_wp_error($option_term) && isset($option_term->name)) :
                                                                                $term_name = $option_term->name;
                                                                        else :
                                                                                $term_name = $option;
                                                                        endif;
                                                                        ?>
                                                                        <?php echo '<option value="'.$option.'">'.$term_name.'</option>'; ?>
                                                                <?php endforeach; ?>
                                                        <?php endif;?>
                                                </td>
                                                <?php endif;?>
// End custom code
                                        </tr>
                <?php endforeach;?>
                                </tbody>
                        </table>
                        <div class="single_variation_wrap" style="display:none;">
                                <div class="single_variation"></div>
                                <div class="variations_button">
                                        <input type="hidden" name="variation_id" value="" />
                                        <div class="quantity"><input name="quantity" value="1" size="4" title="Qty" class="input-text qty text" maxlength="12" /></div>
                                        <button type="submit" class="button alt"><?php _e('Add to cart', 'woothemes'); ?></button>
                                </div>
                        </div>
                        <div><input type="hidden" name="product_id" value="<?php echo esc_attr( $post->ID ); ?>" /></div>
                        <?php do_action('woocommerce_add_to_cart_form'); ?>
                </form>

in which file I need to add the code block?

Could you please tell me where I need to add this code to get a custom text field in products page? Also is it possible include drop-downs? Also I need to have a list of countries and based on the country selected I need the networks to be listed. I want to know where the code for this goes?