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.