domain keys

Greetings:

I would like to use / modify exiscan to scan an entire message so it 
can be signed with a domain key.  Is there an exiscan plugin spec, or 
do I need to make mods to the existing changes that the exiscan patch 
made?  I figure if every message is being scanned for spam / viri 
etc... I should be able to sign the message with a domain key at the 
same time without performing a separate scan.

Any advice is greatly appreciated.

Kindest Regards,

Jerry