Not a high priority but currently there is no facility to create custom headers beyond X-No-Archive and Organisation.
That is correct. If you let me know what custom header types you want to include, I'll add them in the next release.
I was thinking more the ability to create your own. MacSoup and others have this. It's trivial but can be useful.
But is also prone to mistakes if you don't know what you are doing. That is why I prefer not to have the field complete open.