S/MIME and disclaimer

Is there a way to add a generic disclaimer to S/MIME mail messages after the
message has been encrypted / signed?
This could happen e.g. on the gateway level...
Is there a way to extend multipart MIME messages? 
Is there a way to add scoping to an S/MIME MIME type? Otherwise text added
after the S/MIME operations would invalidate the signature.
Would a disclaimer added after signing be readable on the receiver side?

Tx,

Jan