Insert email headers into custom fields?

There isn’t a built-in way, but you can do that by writing a “postie_post” filter. See: http://postieplugin.com/extending/

http://postieplugin.com/postie-polylang/ is a full example, although it dosen’t do exactly what you want.
If you’re willing to pay me I could help you. Contact me at [email protected] if interested.