Outlook 2010 - Automatic signatures for auto-mapped shared accounts

we have Exchange user profiles set up so that they have a personal mailbox and then we auto-mapped shared mailboxes to them.

The problem is that this way apparently it is not possible to set separately default signatures for each mailbox.

Before, we manually added an account for each shared mailbox, so you could chose a different default signature for each account:

But now there is just one account, the mapped accounts do not appear in the drop down list in the Signature settings. Is there a way to still set a different signature for each mailbox with this configuration?

Thank you

Paolo