In the Zimbra webmail interface, in Preferences/Accounts, there's an "Add External Account" button, but in Desktop, there's not. It just says "To further manage this offline account, please visit the Setup Page", which implies that there isn't a way to add another account, or at least *doesn't* imply that that's where you would do it, however if you actually do visit the setup page, you find the option to add one.
That message would imply to me that you visit the Setup page to make additional account changes. If you feel that the wording is not correct and could be misunderstood then file a RFE on how you would like it to be changed. The Zimbra project team will then decide whether to do it or not.
Reference: Bug 29921 - Non-intuitive location to add another mail account
Requesting setupOfflineAccts to read "To further manage this and other offline accounts, please visit the Setup Page."