Seperate Session Key and Encrypted Data

[Christian Loehle]

I want to use gpg to encrypt a potentially large file to some
(cloud-like) storage provider, the recipients are not known at the time
of uploading.
What I want to do is to send the encrypted session key of the file to a
recipient, when I 'add' them, without re-uploading or even touching the
original (encrypted) file.
This should be possible, does anyone know how to? I'm also open to other
suggestions.