Malthe suggested that your "export as XML" and not as ZEXP. ZEXP is a binary format native to Zope. You can read it with Python but then you can read the ZODB directly with Python as well.
Just because your application also keeps application data in Postgres does not mean that all data such as your users is kept in Postgres. One would hope that your application is sufficiently well documented to explain which data is not stored in the ZODB as, except in rare cases such as RelStorage, the majority of data of a Zope server will be stored in the ZODB.
Finally, I must point out that Zope 2.6 is a very old and no longer supported release.