SMF doesn't support adding exceptions, unfortunately.
I only blocked one exit node (the largest one, blutmagie). It's funny how many people use that one exit node. People complain about Bitcoin being too centralized...
Okay, will just make sure I'm not exiting through it when sending PMs...
P.S.:
From where I am, blutmagie is a fairly decent node, high performance and in a far away jurisdiction.