I'm not sure I understand quite what you are asking Zimbra to do.
Do you in essence want a user to have multiple user-ids with which to log into Zimbra, but they all really just use the same mailbox/Zimbra account?
For example, do you want it set up so user MikeB@MYU.org can log into Zimbra as CS101MikeB, or LIBIDMikeB or MikeB, but they are all the same account?
If so, then yes Zimbra can do that with no problem using aliases. When an account has aliases, any of the aliases can be used to log into that account.
If authenticating with an external LDAP (Active directory in my case), the actual account owner ID (MikeB@MYU.org in my example) is the one sent to the Active Directory for authentication, no matter which alias logged into Zimbra. Likewise, if using another client (Thunderbird in my case) to search the Zimbra LDAP, using any of the aliases will return the master email address.
Production: 5.0.18_GA_3011.UBUNTU8 FOSS
Ubuntu 8.04 LTS
Testing: 7.1.4_GA_2555.UBUNTU10_64 NETWORK edition
Ubuntu 10.04.4 LTS