Hi!
I think that it's not possible to "filter" the forwarding address in the way you need it..
You can however disable the option to set a forwarding address via the Zimbra Administration Console at COS level (Select the COS, enter the "Features" tab and uncheck the "Allow users to specify a forwarding address" option and press Save), and then set a "hidden to the user" forward address that can be changed only by and admin.
Hope this helps,
Cine - Community Manager - ZeXtras
www.zextras.com