I'd like to be able to run Zimbra on Gentoo, but one thing I don't understand is why Zimbra requires it's own build of many Third Party software packages.

Is there a way to build Zimbra with the third-party packages that I have already installed?

I've put some research into the availability of third party packages and they all appear to exist.
Code:
#############################################################################
#
# Build 3rd Party Products
# http://varlogmessages.vroomvroom.org/documentation/ubuntu-port/ubuntu-build-instructions/ubuntu-build-thirdparty
#
# assumptioin: sleepycat = berkley db = sys-libs/db
# masked: app-text/xml2 ~x86
# it is possible that net-mail/mailgraph is required
# www-servers/nginx triggered a file collision on
#   /usr/lib/perl5/5.8.8/i686-linux/perllocal.pod
# packages in portage
emerge --pretend dev-libs/openssl sys-libs/db dev-db/mysql app-text/xml2 app-crypt/heimdal net-misc/curl dev-libs/cyrus-sasl net-nds/openldap app-text/aspell app-antivirus/clamav www-servers/apache dev-lang/php mail-mta/postfix mail-filter/dspam net-analyzer/rrdtool net-analyzer/net-snmp net-misc/memcached www-servers/nginx

# packages in swegener overlay
emerge --pretend --verbose app-portage/layman
layman -a swegener
emerge --pretend --verbose dev-util/google-perftools
The output of emerge --pretend is:
Code:
[ebuild     U ] dev-libs/openssl-0.9.8g-r2 [0.9.8g]
[ebuild   R   ] sys-libs/db-4.5.20_p2
[ebuild     U ] app-antivirus/clamav-0.94 [0.93.3] USE="-test%"
[ebuild   R   ] app-text/xml2-0.4
[ebuild   R   ] app-text/aspell-0.60.5
[ebuild   R   ] net-analyzer/rrdtool-1.2.27
[ebuild   R   ] net-misc/memcached-1.2.5
[ebuild     U ] net-nds/openldap-2.3.43 [2.3.39-r1]
[ebuild   R   ] dev-db/mysql-5.0.60-r1
[ebuild   R   ] net-analyzer/net-snmp-5.4.1.1
[ebuild   R   ] www-servers/nginx-0.6.29
[ebuild     U ] net-misc/curl-7.18.2 [7.15.1-r1] USE="-libssh2% -nss%"
[ebuild   R   ] dev-libs/cyrus-sasl-2.1.22-r2
[ebuild     U ] www-servers/apache-2.2.9-r1 [2.2.9]
[ebuild     U ] mail-mta/postfix-2.5.5 [2.2.10] USE="-dovecot-sasl%"
[ebuild   R   ] app-crypt/heimdal-0.7.2-r3
[ebuild  N    ] mail-filter/dspam-3.8.0-r13  USE="clamav daemon ldap mysql syslog user-homedirs -debug -large-domain -postgres -sqlite -virtual-users"
[ebuild   R   ] dev-lang/php-5.2.6-r6