Header fields per Identity?

Hello all,

I like to put the resource of my gpgkey (which is not listed on a
publickeyserver) in the mail header.

KMail can do this already since I can define Fields and its values.
But how can I set the value by the Identities  instead of globaly?
Guess the reason is clear when I have 2 or more mail accounts so I can
not have one global resource for my gpg key in the header since most
likely, the different identities have different keys.

Thanks a lot for any hint.
Michael