Welcome to the forums,
The short answer is no, you can't separate the web-interface from the rest of the package. The Zimbra Collaboration Suite uses it's own customized instances of a serious collection of software packages - OpenLDAP, MySQL, Apache, Tomcat (4.5 prior) Jetty (5.0), Postfix, Lucene, Verity, ClamAV, SpamAssassin, DSPAM (though off by default) AMaViS/Amavisd-new, Aspell, James/Sieve, Perdition (4.5 prior), NGINX (5.0), etc.
However, if you'd like to have it grab mail from your exchange setup individuals can setup individual external IMAP/POP account aggregation, or you could use fetchmail/webmin to do so; but that's not really the normal setup.
For transition purposes from you exchange solution you can run it as a
Split Domain - Zimbra :: Wiki
There's also an exchange migration wizard in your Admin Console GUI downloads area (http:// server.domain.com/downloads/index.html)
I also encourage you to checkout
Zimbra Desktop which uses the same AJAX interface & can be connected to a Zimbra server or any IMAP/POP account. (ie: your exchange accounts)