Add content to the content

Hi,
I have looked for the answer to this but failed. I wonder if it’s possible to add content to the content on single.php and page.php?

I want to add a div right at the beginning of the content. I have done it with a custom hook and it works but I want to do it without having to add to the functions.php and the templates. I’d also like to do it without having to add a shortcode in the the editor. I’m using custom fields to determine what the content in the div (if any) will be.

The exact spot I want my additions and where I now have my custom hook is here (in single.php page)

<div class="entry"><?php
custom_hook(); ?>