When customers migrated from hosted Zimbra, they ran a hybrid deployment (i.e. ran directory services in parallel with Office 365.)
Typically, both legacy and target servers were in the same domains. That is, until we received a request from a client who wanted to migrate from Zimbra to Office 365 AND change the domain name.
In zCalReader 6.2.03 and above they can.
Note: You should still use your Zimbra Admin credentials for the Zimbra side.