It only returns a subset of user data https://github.com/NodeBB/NodeBB/blob/master/src/posts/user.js#L24-L28
If you want to add more you can do so in a plugin, using the hook filter:posts.modifyUserInfo
Is it possible to include additional metadata into a post? I am looking to have users write game reviews on my site, and include some sections with key information for searching and creating embeds. The more customizable, the better, but I would ultimately want something like:
Nope. You would need to make a plugin. There's no way to add any additional data to the post without writing your own functions to handle the submit action.