Assuming you just want to add some content via a simple template file you can add the following in your theme `layout/local.xml`

    <default>
        <reference name="content">
            <block type="core/template" name="custom.content" template="my/custom/template.phtml" />
        </reference>
    </default>

Then you just need to create your `template/my/custom/template.phtml` file with the content you need inside it