Hi AMT12,

It’s not available for Office 365 Plan P1. An alternative is to make signature in Outlook or OWA on a per user basis. For Office 365 enterprise, to automatically apply disclaimers to e-mail messages, you use transport rules. You create disclaimers in the Exchange Control Panel. For the detailed information, see Add Disclaimers to Messages.
http://help.outlook.com/en-us/140/ff852816.aspx

Thanks,
Monica Tong