Add Content to Content()

// Hook into the_content filter here
function append_to_the_content($content){
    ob_start();
    // Start doing stuff here ...

    // End doing stuff here ...
    $new_content = ob_get_clean();
    return $content.$new_content; // Append new content
    return $new_content.$content; // Prepend new content
} // function append_to_the_content($content)
// 11 priority avoid wpautop() that messes custom HTML
add_filter('the_content', 'append_to_the_content', 11);

Regards.