Can you Edit the Post Screen

These two links will give you some background information on:
a) How to add a meta box to the post screen.
b) How to save the data to the post meta.

https://codex.www.ads-software.com/Function_Reference/add_meta_box
https://thinkvitamin.com/dev/create-your-first-wordpress-custom-post-type/

There are lots of posts out there on different ways to add meta boxes etc. I just the first ones I came across. If you are still having problems let me know how far you got, and what you still need help with.