single custom field for posts

Hi,
I’m aware of native custom fields for posts – the problem is that they are text boxes (both the key and the value), which will not really work for what I’m trying to do.

I’m also aware that what I’m trying to do is very simple with ACF.

What I’d like to do is have one custom field for all posts type ‘post’ which would allow you to select a custom post type. So when I create a post, you’ll also choose one custom post type to be added (to the sidebar).

So is it possible to beef up the native custom field to make it a select box (As opposed to a text box) and let is search through the custom type posts? Or somehow modify the admin post creation dashboard to add it without installing another plugin?

I know that it’s kind of reinventing the wheel as what I need is very simplified ACF but I’m trying to add it to a theme (custom post creation, adding the custom post selector in posts dashboard) without the need to add plugins.

Thanks