I believe this is what you're looking for:
Bug 14426 – Save e-mail as template option
The feature has been requested some time ago. You can check pm.zimbra.com to see if they are planning on including it in a future release, but I'm not aware of a timeframe.
Adding signatures in the web client that you can select can work in some situations as a workaround. There's also a possible solution listed in the bug that may work for you as well.