No this not work with zimbra 7 and 6 (only 5).
Now, with Zimbra 6.4 and after, you can set a default signature with zmprov commands:
* zimbraDomainMandatoryMailSignatureEnabled (TRUE/FALSE) TRUE enables this feature.
* zimbraDomainMandatoryMailSignatureText. This creates the plain text version.
* zimbraDomainMandatoryMailSignatureHTML. This creates the HTML version.
References:
[SOLVED] Domain Disclaimer for Zimbra 7.1.0