I am trying to customize the user form to my needs by adding specific fields and have edited the user.edit.php(which I don’t think is the best method). I want to add a structure field, which I realize is not allowed. How can I add fields to the user form in the panel?
Unfortunately at the moment there is no “official” way to add custom fields for users. (and I don’t know an “unofficial” way …)
I would also need this feature. Unfortunately this is a dealbreaker using kirby for a simple blog.
“Flexible as hell” should be starred 
This will be a feature in the upcoming Kirby 2.2 release. See the commit: https://github.com/getkirby/panel/commit/5add0eefa8779baec7f87f3405e2cc871b62c633