"Diverting from the Official Tree" gives you some tips and tricks on how to use your own Portage tree, how to synchronise only the categories you want, inject packages and more. 1.6 2007-02-26
Using a Portage Tree Subset Excluding Packages/Categories

You can selectively update certain categories/packages and ignore the other categories/packages. We achieve this by having rsync exclude categories/packages during the emerge --sync step.

You need to define the name of the file that contains the exclude patterns in the --exclude-from variable in your /etc/make.conf. 

PORTAGE_RSYNC_EXTRA_OPTS="--exclude-from=/etc/portage/rsync_excludes"

games-*/*

Note however that this may lead to dependency issues since new, allowed packages might depend on new but excluded packages.

Adding Unofficial Ebuilds Defining a Portage Overlay Directory

You can ask Portage to use ebuilds that are not officially available through the Portage tree. Create a new directory (for instance /usr/local/portage) in which you store the 3rd-party ebuilds. Use the same directory structure as the official Portage tree!

Then define PORTDIR_OVERLAY in /etc/make.conf and have it point to the previously defined directory. When you use Portage now, it will take those ebuilds into account as well without removing/overwriting those ebuilds the next time you run emerge --sync.

Working with Several Overlays

For the powerusers who develop on several overlays, test packages before they hit the Portage tree or just want to use unofficial ebuilds from various sources, the app-portage/gentoolkit-dev package brings you gensync, a tool to help you keep the overlay repositories up to date.

With gensync you can update all the repositories at once, or select just a few of them. Each repository should have a .syncsource file in the /etc/gensync/ configuration directory which contains the repository location, name, ID, etc.

Suppose you have two additional repositories called java (for the in-development java ebuilds) and entapps (for the applications developed in-house for your enterprise). You can update those repositories with the following command: 

# gensync java entapps
Non-Portage Maintained Software Using Portage with Self-Maintained Software

In some cases you want to configure, install and maintain software yourself without having Portage automate the process for you, even though Portage can provide the software titles. Known cases are kernel sources and nvidia drivers. You can configure Portage so it knows that a certain package is manually installed on your system. This process is called injecting and supported by Portage through the /etc/portage/profile/package.provided file.

For instance, if you want to inform Portage about gentoo-sources-2.6.11.6 which you've installed manually, add the following line to /etc/portage/profile/package.provided: 

sys-kernel/gentoo-sources-2.6.11.6