The only way that I can think of to get this type of setup working where you have multiple AD servers with multiple domains that you want users to just login as "user" and all be in the same domain in Zimbra would be to merge all the AD servers into a single virtual server under one domain, then use that as an external Authentication source.
Browsing the forum for something else I saw mention of this product:
Penrose which might do that...
Understand though that I have no experience with any of this and am jsut guessing that this might work...