Encrypting to a sign-only key // Is decryption possible?

[vedaal at nym.hush.com]

I remember once doing this by mistake, using one of the old pgp command lines (? 6.x), where it allowed me to encrypt to a person's signing key.

In the event that such a thing occurs, could gnupg decrypt it?
(Is there a way for gnupg to make one's signing key, a sign and encrypt key, AFTER it was generated as 'only-sign' ? )

TIA,

vedaal