existing keys as subkey

[1984]

Hello,
I want to install a pgp/gpg secured mailinglist. The best possibility
is to generate a new key, includes every key of the entered users.
Every mail to the list must be encrypted with this public key.
My question is: how can I build a key and implement other existing
keys as subkeys of this one? So that every mail encrypted by the key
is encrypted by the keys of all users.

The only way, I think, is to use ADK of PGP, because in GnuPG you
cannot take existing keys as a subkey, you can only generate a new
one.

bye Sören