I have PGP Desktop 9.0.2 on an Apple 10.4.2

As I understand it, the "Opportunistic Encryption" Policy will Sign and
Encrypt emails when it can find a receipients key. This I sorta like.
But if it cannot find a key, it not only sends in the clear but does
not sign.

How can I have a sudo "Opportunistic Encryption" Policy that will sign
and encrypt (as is normal behavior when receipient key found), but if
no receipient key found, sign all anyway, then send in the clear?