You Need to create one small module for this.First of all create a file in app/etc/module folder.lets say file name is yourpackagename_yourmodulename.xml and the following content in this file.
<?xml version="1.0"?>
<config>
<modules>
<Yourpackagename_Yourmodulename>
<active>true</active>
<codePool>local</codePool>
</Yourpackagename_Yourmodulename>
</modules>
</config>
Now create config.xml file in app/code/local/Yourpackagename/Yourmodulename/etc folder and following code in config.xml file.
<?xml version="1.0"?>
<config>
<modules>
<Yourpackagename_Yourmodulename>
<version>0.0.1</version>
</Yourpackagename_Yourmodulename>
</modules>
<global>
<helpers>
<yourmodulename>
<class>Yourpackagename_Yourmodulename_Helper</class>
</yourmodulename>
</helpers>
</global>
<adminhtml>
<acl>
<resources>
<admin>
<children>
<system>
<children>
<config>
<children>
<yourmodulename_options>
<title>some text</title>
</yourmodulename_options>
</children>
</config>
</children>
</system>
</children>
</admin>
</resources>
</acl>
</adminhtml>
</config>
Now create system.xml file in app/code/local/Yourpackagename/Yourmodulename/etc folder
and following code in config.xml file.
<?xml version="1.0" encoding="UTF-8" ?>
<config>
<tabs>
<yourmodulename translate="label" module="yourmodulename">
<label>some text</label>
<sort_order>999</sort_order>
</yourmodulename>
</tabs>
<sections>
<yourmodulename_options translate="label" module="yourmodulename">
<label>some text</label>
<tab>yourmodulename</tab>
<frontend_type>text</frontend_type>
<sort_order>1000</sort_order>
<show_in_default>1</show_in_default>
<show_in_website>1</show_in_website>
<show_in_store>1</show_in_store>
<groups>
<yourmodulename_links translate="label">
<label>Cron</label>
<frontend_type>text</frontend_type>
<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>
<fields>
<cronexpr>
<label>cronexpr</label>
<frontend_type>text</frontend_type>
<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>
</cronexpr>
</fields>
</yourmodulename_links>
</groups>
</yourmodulename_options>
</sections>
</config>
Now create Helper folder in app/code/local/Yourpackagename/Yourmodulename/Helper and in this folder create Data.php with following content
<?php
class Yourpackagename_Yourmodulename_Helper_Data extends Mage_Core_Helper_Abstract
{
}
?>
now refresh site and clear cache and go to admin you will see new config created and put your cron value their and use the saved value with the foloowing code wherver you need.
<crontab>
<jobs>
<stock_setcron>
<schedule>
<cron_expr>yourmodulename_options/yourmodulename_links/cronexpr</cron_expr>
</schedule>
<run><model>stock/cron::setcron</model></run>
</stock_setcron>
</jobs>
</crontab>