Currently, it is not possible to synchronize unlicensed shared mailboxes from the o365 environment into the control panel. This can be justified by the fact that only licensed mailboxes are intended to be covered by our products.
Create a secondary environment (Service Dashboard -> Secondary Environments) that points to the same target as the primary environment.
Then create the mailbox manually in the Control Panel (Customer Settings -> Mailboxes) using the new secondary environment as the environment.
The mailbox will now be able to use the archive and the signature and disclaimer module and will be filtered by our spam filter.