Seems basic but the part that is not in black and white when searching this hook is how to set up the theme render element.
Below is about all the help I could find, but looking into the field.module I was able to figure out how to make a theme function for my custom element.
<?php
function hook_node_view($node, $view_mode, $langcode) {
$node->content['my_additional_field'] = array(
'#markup' => $additional_field,
'#weight' => 10,
'#theme' => 'mymodule_my_additional_field',
);
}