[Evolution] Setting headers in incoming mail

[Peter Gordon <peter pg-consultants com>]

I have a message filter, which is a pipe to a command, which does extra
checking for spam. Having checked, I would like to force this mail to
the "Junk" folder. When creating the rule, the "Junk" folder cannot be
chosen.

As I workaround, I have created a different folder, and I sent the mail
to that, but I am curious as to how to select the normal "Junk" folder.

A second question: How do I add X-headers to incoming mail?

Thanks,

Peter