Adding headers in the router?

[Arnt Gulbrandsen <agulbra@troll.no>]

I can do lots of neat stuff in the router, but can I add headers?  I'm
talking about an X-header, not one of the standard ones.

(For background: What I want is to add a "this mail is (maybe) from a
customer" header to inbound mail.)

--Arnt