Isn't server-server interop documented in NE admin guide?
pp. 70 & 75.
Also
Free Busy Interop for Exchange - Zimbra :: Wiki Exchange Freebusy Interop - Zimbra :: Wiki Sumatra's Calendar Server Blog: Exchange 2007 - Zimbra Free/Busy: The Magic PowerShell Command Are you free or busy? » Zimbra :: Blog http://www.zimbrablog.com/blog/wp-co...d-exchange.pdf
Some of the above links are more command-line oriented but it appears that you can do most or all of what's needed using the web admin interface these days (see the admin guide). You do apparently need the cooperation of the Exchange admins as it sounds like they need to give you a "foreign principal" account as well as access to their AD or Global Catalog.