This can be done a few ways. As mentioned by adol, the Ubuntu wiki has a nice example of how to do this by creating a local repository with mini-dinstall and adding that to your pbuilder config. Dennis' answer about using dpkg-scanpackages works as well.
I've been doing this recently with apt-ftparchive. I like this approach since I find it very light weight. Here's annotated example of what I do:
# From my ~/.pbuilderrc file
# Location of the dir where you keep pbuilder hook scripts.
HOOKDIR="/home/andrew/.pbuilder-hooks"
# Path to your local repo to be used as a mirror written as apt source line.
OTHERMIRROR="deb file:///home/andrew/pbuilder/local_repo ./"
# Path to your local repo. This tells pbuilder to mount this directory so it is available in the chroot.
BINDMOUNTS="/home/andrew/pbuilder/local_repo"
# As we need to have the apt-ftparchive command, we need to insure this package is installed.
EXTRAPACKAGES="apt-utils"
You also need a pbuilder hook:
# From my ~/.pbuilder-hooks/D5update-local-repo file
# Path to the local repo.
LOCAL_REPO="/home/andrew/pbuilder/local_repo"
# Generate a Packages file.
(cd $LOCAL_REPO ; apt-ftparchive packages . > Packages)
# Update to include any new packages in the local repo.
apt-get update
Now all you have to do is drop the packages into your local repo and they will be available to pbuilder. If you are trying to chain build a string of dependencies you can make you pbuilder results directory as your local repo directory.
You can probably imagine other variations on this. For instance, you could use dput with a post_upload_command to generate the Packages file instead of using the hook.
This Debian wiki page could also be helpful.