how can I sign rpms in koji?

[Mátyás Selmeci]

I have imported several rpms into our koji without realizing that they 
were unsigned. I'd like to sign them with our gpg key, but I can't 
figure out how to do that after the fact. We use the signing plugin from 
https://fedorahosted.org/koji/ticket/203, but that only works for rpms 
we build ourselves.
--
-Mat