Rt-mailgate requestor/creator problem

I am trying to use a Thunderbird extension to redirect mail into RT via
rt-mailgate. The extension adds a “Redirected-From:” field to the header
of the message. I would like the creator of the ticket to be determined
from the “Redirected-From:” while the requestor is detrmined from the
standard “From:” field.

I can work out how to parse the header and extract the "Redirected-From:"
user, but how do I set different ticket creator and requestor using