Currently (CU3), you only can create new groups which are immediately mail-enabled. But you cannot select groups to mail-enable them. You have to do this via shell.
Users can manage Distribution groups, but can only add members who have an AD object.
For example : Add a second multi-value attribute (eg externalMembers) and a permissions mask to provide the ability for an administrator to allow certain DGs to have a secondary list of external SMTP addresses.
Why not make actual "automatic" out of office replies based on user connectivity and automatically forward to any of the following:
> The distribution group the user belongs to (Small groups)
> User Selected member(s) of his/er distribution group (Larger Groups)
> User Selected member(s) (Non grouped users)
> Administrator Selected member(s) (Non grouped users)