Magento add custom textbox,select box to existing system -> configuration tab.

if we want to add custom fields like textbox,select box to existing system -> configuration tab -> catalog.

<?xml version="1.0"?>
<config>
   <sections>
        <catalog>
            <groups>
                <frontend>
                    <fields>
                        <mayankpatel translate="label">
                            <label>Custom Textbox Value</label>
                            <frontend_type>text</frontend_type>
                            <source_model>adminhtml/system_config_source_yesno</source_model>
                            <sort_order>1</sort_order>
                            <show_in_default>1</show_in_default>
                            <show_in_website>1</show_in_website>
                            <show_in_store>1</show_in_store>
                        </mayankpatel>
                      </fields>
                </frontend>
            </groups>
        </catalog>
    </sections>
</config>