Hi Petri,
You need access to a a user mailbox to be able to set OOF. OOF is per-user and in Exchange 2007, we do not have a way to let 'helpdesk' set OOF on behalf of a user.
To answer your 2nd part of the question:
We do have options you are mentioning. An end-user controls whether to send OOF or not (please refer to the UI in OWA/Outlook).
Internal OOF is sent by default when the end-user turns on OOF. External OOF is optional for the end-user.
So, a user can be in one of the following 4 states:
(1) No OOF
(2) Send OOF responses to messages received from Internal users only
(3) Send OOF responses to messages received from internal users and external users that are part of the user's known contacts
(4) Send OOF responses to messages received from internal users and all external users