I'm curious if there is a way to set up a user in zimbra and either; configure the user to use local authentication instead of the LDAP auth that is configured for the domain. Or have zimbra check local auth if the user does not exist on LDAP.

The problem is creating an LDAP account gives other access that this type of user should not have. And creating user classes in LDAP (the right way) is a big undertaking for this corner case compared to tweaking zimbra to use local authentication for specific users.