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.
# 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.
# As we need to have the apt-ftparchive command, we need to insure this package is installed.
You also need a pbuilder hook:
# From my ~/.pbuilder-hooks/D5update-local-repo file
# Path to the local repo.
# Generate a Packages file.
(cd $LOCAL_REPO ; apt-ftparchive packages . > Packages)
# Update to include any new packages in the local repo.
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.