The technique suggested above, which uses Clone and AddExistingMessage, will only work within the currently logged in users account. It will not work when proxied to another account. This is because both methods/abojects are part of the GroupWise object API, which is an account-based API. The Account object referred to in the code is actually the account of the currently logged in user.
The only workaround would be to use the multilogin method. This method gives you a new account object representing another users account. You need to know the user name and password of the other user to use this method.
You can access another users account object by doing something like this:
set iAccount = GroupWise.Session.MultiLogin(UserID,,Password)
UserID and Password are both strings.
I hope this helps.